Changes in Thyroid Hormone Levels Within the Normal and/or Subclinical Hyper- or Hypothyroid Range Do Not Affect Circulating Irisin Levels in Humans

Abstract

Background:

Both thyroid hormones and irisin increase energy expenditure and induce browning of adipose tissue. However, irisin physiology and regulation remain largely unknown, and existing data are mainly derived from observational studies. In this study, we aimed to elucidate whether changes in thyroid-axis hormones alter circulating irisin levels in humans, thereby exerting a direct downstream effect on serum irisin.

Subjects and methods:

Samples from a cross-sectional evaluation and two interventions were utilized, including patients who had previously undergone thyroidectomy. In the cross-sectional study, 96 consecutively enrolled subjects were divided into a euthyroid group and a subclinical hyperthyroid group, according to their serum thyrotropin (TSH) levels (TSH cutoff 0.3 mIU/L). In interventional study A, 34 patients who had undergone thyroidectomy due to thyroid cancer were withdrawn from their thyroxine replacement treatment for five weeks. In interventional study B, 13 patients underwent a recombinant human TSH stimulation protocol, and blood samples were drawn at baseline, day 3 (i.e., at least 24 hours after the second intramuscular injection), day 5, and day 10.

Results:

Irisin concentrations were not associated with thyroid-axis hormones (i.e., TSH, free thyroxine, and free triiodiothyronine) cross-sectionally in either the overall cohort or in the euthyroid and/or subclinical hyperthyroid subgroups (p > 0.05). There was no significant difference between euthyroid and subclinical hyperthyroid subjects (p = 0.60). Levothyroxine withdrawal did not result in any changes in irisin concentrations (p = 0.33). Recombinant human TSH stimulation did not induce any significant changes in circulating irisin (p = 0.60).

Conclusions:

Changes in thyroid-axis hormone levels within the physiological or supraphysiological range do not affect circulating irisin levels in humans. Therefore, their metabolic effects are most likely independent of each other. Other regulators of irisin levels should be identified in the future.

Introduction

Targeting energy expenditure to reduce body weight through induction of brown fat has been proposed as a new method in the fight against obesity (1). Thyroid hormones are known to increase energy expenditure (2) and have been used toward this end in the past (3). Recently, it was shown that activation of thyroid hormone receptors induces browning of white fat, accompanied with a marked increase in Uncoupling Protein 1 (UCP1) gene expression (4).

A novel myokine termed irisin has also been proposed to act as an agent that increases energy expenditure through UCP1 induction, and it leads to reduction of insulin resistance and weight loss (5), thereby raising high expectations about its possible role in tackling metabolic disorders (6). However, little is known about the physiology and regulation of irisin in humans. Given its proposed beneficial action, it is of great importance to assess physiological mechanisms that regulate irisin levels.

Although it was originally suggested that irisin levels increase after exercise (5,7), this finding has been challenged by other investigators (8). It has previously been shown that coffee consumption, also known to increase energy expenditure, is not associated with higher irisin levels (9). In addition, previous cross-sectional studies reported associations between irisin and anthropometric or biochemical parameters (10 –12), other myokines (13), and adipokines such as adiponectin (14). Studies in children and adolescents have shown that irisin is regulated in a sex-dimorphic manner (15), is positively associated with body mass index (BMI) and leptin (16), and is increased after a one-year life-style intervention program that resulted in significant weight loss and improvement of other clinical parameters (17). In a longitudinal interventional study, irisin levels were higher in obese children with impaired glucose metabolism, and irisin was significantly associated with pubertal stage, being lower in prepubertal children compared with pubertal ones (18). Yet, there are no published data on associations of thyroid state in children and/or adolescents—a critical age for changes in thyroid axis and development of autoimmune thyroid diseases—with irisin concentrations.

On the other hand, irisin was not associated with thyrotropin (TSH) in adult euthyroid subjects with mild hypercholesterolemia (19), and obese or anorexic patients (20). However, a study including patients with hyperthyroidism and hypothyroidism showed that irisin levels were negatively associated with TSH and creatine kinase (CK) levels, and positively associated with free thyroxine (fT4) levels (21). More recently, circulating irisin was found to be increased in both experimentally induced hypothyroid and hyperthyroid rat models in correlation with elevated CK levels, and therefore was associated with thyroid state-induced muscle damage (22,23).

Despite the previous purely observational data linking thyroid-axis hormones and irisin, interventional human studies that would support a causal effect of TSH or peripheral thyroid hormones on irisin concentrations are currently missing. Therefore, it remains unknown whether the effects of thyroid-axis hormones on metabolism could possibly be mediated by irisin. Samples were thus utilized from a series of both cross-sectional and interventional human studies in order to elucidate the downstream effect of subclinical and experimentally altered thyroid state on serum irisin levels.

Subjects and Methods

Studies were conducted at the Department of Endocrinology—Endocrine Oncology, Theagenio Cancer Hospital, Thessaloniki, Greece, and at the Department of Endocrinology and Diabetes, Hellenic Red Cross Hospital, Athens, Greece. Samples from the interventional studies were initially collected in the context of diagnosing recurrence of thyroid cancer via measurements of thyroglobulin/and or whole-body scan using routine standardized protocols. After completion of the protocol and under the written approval of the local ethics committees and the patients' themselves, serum aliquots were kept for the purposes of this study. All participants provided written informed consent. Serum samples were shipped to the Mantzoros Laboratory, Division of Endocrinology, Diabetes, and Metabolism, Beth Israel Deaconess Medical Center (BIDMC), Boston, MA, for the quantification of irisin. The study protocols were approved by the Theagenio Cancer Hospital Scientific Committee, the Hellenic Red Cross Hospital Scientific Committee, and the BIDMC Institutional Review Board, and were in accordance with the Declaration of Helsinki.

Cross-sectional study

Ninety-six patients with a history of multi-nodular goiter or Graves' disease were originally recruited, as previously described (24). All participants had undergone total thyroidectomy, and histology was negative for thyroid cancer. Subjects had been under replacement therapy with levothyroxine for a period of at least one year, and were periodically followed up to avoid abnormal variation of TSH and thyroid hormone levels. Exclusion criteria included hypothyroid patients, history of previous cancer, and debilitating chronic diseases. Patients were examined by an experienced physician, underwent standard laboratory examinations, and completed a demographic questionnaire. After laboratory tests, subjects were divided into two groups by using a TSH cutoff value of 0.3 mIU/L (normal range 0.3–4.0 mIU/L): 80 subjects were classified as euthyroid (TSH range 0.3–3.28 mIU/L), and 16 subjects were classified as subclinical iatrogenic hyperthyroid (TSH <0.3 mIU/L). Early morning blood samples were collected after an overnight fast and prior to thyroid hormone ingestion (24). After collection, samples were immediately centrifuged and stored at −70°C until shipment.

Interventional study A

Thirty-four patients (7 males) who had previously undergone thyroidectomy with histologically proven differentiated thyroid cancer were prepared for thyroglobulin measurement and/or a whole-body scan under a standardized protocol (25). Briefly, thyroxine treatment was stopped and triiodothyronine (T3) was administered for three weeks and then stopped for two weeks (the total duration of the protocol was five weeks). In this way, iatrogenic hypothyroidism was induced. Blood samples were collected in the early morning, after an overnight fast and prior to thyroid hormone ingestion for the measurement of TSH, fT4, total T3, irisin, and CK at baseline and at the end of the study period. After collection, samples were immediately centrifuged and stored at −70°C until shipment.

Interventional study B

Thirteen patients (5 males) with a history of thyroid cancer and total thyroidectomy underwent their annual follow-up with a neck ultrasonography and rhTSH test (recombinant human thyrotropin-α, Thyrogen^®; Genzyme Corp., Cambridge, MA) while treatment with thyroxine was continued, and fT4 levels remained essentially unaltered. rhTSH (0.9 mg) was administered by intramuscular injection for two consecutive days, as appropriate (26). It is known that the sodium–iodide symporter (NIS) is not expressed in muscle tissue (27), and therefore it is unlikely that an alteration in iodide uptake might have had any impact on irisin levels. Moreover, the patients did not receive radioactive iodine for whole-body scan. Blood samples were drawn at baseline (day 1), day 3 (at least 24 hours after the second injection), day 5, and day 10 for the quantification of TSH, CK, and irisin.

Biochemical and hormone measurements

Due to the different centers involved and the independent design of the studies, thyroid-axis hormones were measured using different laboratory methods in each study. However, since the studies are independent from each other, this does not have any impact on the statistical evaluations that were conducted. In the cross-sectional study, TSH, fT4, and fT3 were measured using radioimmunoassay (RIA) as previously described (24). In the LT4 withdrawal study, TSH, fT4, and total T3 were also measured using RIA (BRAHMS, GmbH, Hennigsdorf, Germany). CK was measured by a kinetic enzymatic method with Roche Cobas c311 Autoanalyzer (Cat # 04524977-190; repeatability: 0.5–2.3%; intermediate precision: 1.8–3.3%). In the rhTSH stimulation study, TSH was measured with a chemiluminescence method with a Roche Elexis 2010, and CPK was measured with a Cobas Integra 800 Autoanalyzer (Roche Diagnostics, Mannheim, Germany).

Samples from all studies were assayed for irisin by enzyme-linked immunosorbent assay (ELISA) using the same previously validated kit, which has been independently evaluated both at the authors' laboratory (14) and by other investigators (28), with similar results (catalog no. EK-067-52; range 0.066–1024 ng/mL; Phoenix Pharmaceuticals, Burlingame, CA) with intra-assay coefficient of variation (CV) of 4–6% and inter-assay CV of 8–10% (29). The differences in irisin levels between the rhTSH stimulation interventional study and the previous studies are due to the use of a different lot number of the same ELISA kit (catalog no. EK-067-52; range 0.066–1024 ng/mL; Phoenix Pharmaceuticals). This does not affect the repeated measures analysis of variance (ANOVA) or other statistical tests used herein. All samples were assayed in duplicate, and the mean value was used for the calculations.

Statistical analysis

Statistical evaluations were performed using SPSS Statistics for Windows v17.0 (SPSS, Inc., Chicago, IL). Data are expressed as mean ± standard deviation (SD), unless stated otherwise. Cross-sectional associations of irisin with other variables were calculated using Pearson's correlation coefficients. Differences between euthyroid and hyperthyroid group were tested using an independent t-test or Mann–Whitney U-test, as appropriate. In the levothyroxine withdrawal study, a two-sample paired t-test was performed. Changes in irisin levels after rhTSH stimulation test were tested using repeated-measures ANOVA. Post hoc Bonferroni pairwise analysis followed in order to identify the exact differences between pairs. As irisin has been proposed to be regulated in a sex-dimorphic manner (12,15), all analyses were additionally stratified according to sex. The level of statistical significance was set at 0.05 for all studies.

Results

Cross-sectional and case-control study: associations and comparisons between groups

Characteristics of the cross-sectional study of euthyroid and hyperthyroid subjects are summarized in Table 1. First, an attempt was made to find associations between irisin levels and thyroid-axis hormone levels. In the euthyroid group, none of the associations between circulating irisin and TSH (r = −0.14, p = 0.21), fT4 (r = 0.02, p = 0.86), and fT3 (r = −0.10, p = 0.36) reached statistical significance. Pearson's correlation coefficients in the subclinical hyperthyroid group also confirmed that irisin was not associated with TSH (r = −0.33, p = 0.21), fT4 (r = 0.25, p = 0.36), and fT3 (r = 0.15, p = 0.57). The associations between irisin and thyroid-axis hormones were not significant, even when all subjects were studied together (overall cohort). The study further evaluated for possible differences in irisin levels between euthyroid and subclinical hyperthyroid participants. As expected, there were significant differences in TSH, fT4, and fT3 between the two study groups (Table 1). However, no statistically significant differences were found in circulating irisin levels (p = 0.60). The comparison remained non-statistically significant after adjustment for BMI, while stratified analysis according to sex did not affect the results either.

Table 1.

Cross-Sectional Study Participants and Case-Control Comparison

	Euthyroid group	Subclinical hyperthyroid group	p-Value
Number of participants (% males)	80 (10%)	16 (6.25%)
Age, years	55.94 ± 12.57	55.07 ± 10.96	0.64
Weight, kg	78.05 ± 15.52	69.94 ± 7.7	0.04
Body mass index, kg/m²	29.94 ± 6.02	27.55 ± 3.55	0.04
Waist-to-hip ratio	0.87 ± 0.05	0.86 ± 0.07	0.26
TSH (mIU/L)	1.50 ± 0.87	0.15 ± 0.09	<0.001
fT4 (pmol/L)	14.64 ± 2.56	16.87 ± 2.78	0.002
fT3 (pmol/L)	3.51 ± 0.67	4.05 ± 0.64	0.005
Irisin (ng/mL)	119.57 ± 44.67	106.96 ± 21.98	0.60^a

Statistically significant values are shown in bold.

The comparison remains statistically non-significant, even after adjustment for BMI.

TSH, thyrotropin; fT4, free thyroxine; fT3, free triiodothyronine; BMI, body mass index.

Interventional study A: levothyroxine withdrawal

In the levothyroxine withdrawal study, patients were aged 47.36 ± 12.5 years with a mean BMI of 28.27 ± 5.35 kg/m². The intervention successfully induced a hypothyroid state, as confirmed by changes in TSH, fT4, and total T3 (p < 0.001). However, irisin levels remained essentially unaltered (89.76 ± 42.27 vs. 86.6 ± 39.76 ng/mL; p = 0.33), even after sex stratification. As expected, CK levels were significantly increased after induction of hypothyroidism (p = 0.001; Table 2), with no correlation existing between CK levels and irisin levels at either baseline (r = 0.06, p = 0.74) or after induction of hypothyroidism (r = 0.08, p = 0.66).

Table 2.

Levothyroxine Withdrawal Study: Comparisons Between Baseline and Five Weeks After Levothyroxine Withdrawal

	On thyroxine	Off thyroxine	p-Value
TSH (mIU/L)	0.71 ± 0.7	82.96 ± 29.38	<0.001
fT4 (pg/mL)	14.50 ± 3.38	2.08 ± 1.03	<0.001
TT3 (nmol/L)	2.09 ± 2.19	1.22 ± 0.16	<0.001
Irisin (ng/mL)	89.8 ± 42.3	86.6 ± 39.8	0.33
CK (IU/L)	82 ± 79.5	203 ± 232.3	0.001

Statistically significant values are shown in bold.

Interventional study B: rhTSH stimulation test

Participants of this cohort were aged 52.38 ± 4.32 years with a mean BMI of 28.99 ± 1.75 kg/m². In order to avoid any possible effects of the intramuscular injection on irisin levels, blood samples were not drawn close to any intramuscular injection, at least 24 hours after the second intramuscular injection (day 3). In this study, no significant change in the levels of irisin (p = 0.60) and CK (p = 0.47) were revealed (Table 3).

Table 3.

Recombinant Human TSH Stimulation Test

	Day 1	Day 3	Day 5	Day 10	p-Value
TSH (mIU/L)	0.90 ± 2.1	119.1 ± 60.5	20.4 ± 11.3	6.6 ± 6.8	0.006
Irisin (ng/mL)	215.2 ± 53.4	198.6 ± 48.1	196.1 ± 52.8	209 ± 62.5	0.60
CK (IU/L)	89.7 ± 47.8	92.2 ± 38.6	98.9 ± 41.3	117.3 ± 69	0.47

Statistically significant values are shown in bold.

Discussion

In this series of cross-sectional and interventional studies, the aim was to investigate the possible downstream effect of thyroid axis hormones on irisin concentrations in humans. It is reported that irisin is not associated with any of the thyroid-axis hormones cross-sectionally in either euthyroid and/or subclinical hyperthyroid patients, or in the overall study sample. Novel data are also presented supporting that irisin concentrations are not affected by subclinical or interventional changes of thyroid state in human subjects.

Thyroid hormones target muscle tissue and exert local actions, including differentiation and increase of muscle metabolism (30), as well as induction of expression of various genes (31). In a recent study, it has been shown that brown adipose tissue (BAT) of hyperthyroid subjects presented higher glucose uptake and muscle metabolism compared with that of euthyroid subjects, while TSH was inversely associated with BAT (32). Interestingly, hypothyroidism was also shown to be associated with BAT induction (33). In addition, thyroid hormones as well as the sympathetic nervous system are proposed as endogenous regulators of BAT (34) and consequent heat regulation (3). The authors' group was the first to show that the Fibronectin Type III Domain Containing 5 (FNDC5) gene, the precursor of irisin, is expressed in thyroid tissue (14). However, irisin treatment did not induce any changes in cell proliferation and/or cancer potential in thyroid cell lines (35).

Based on apparent similarities between thyroid hormone and irisin effects on metabolism, it was hypothesized that these effects of thyroid hormones could be possibly mediated by and/or attributed to alteration in irisin concentrations. However, in contrast with a few recent reports (21,22), no significant associations between thyroid hormones and irisin were found in the cross-sectional and case-control studies. These discrepancies may be attributed to the different design of these studies. In their study, Ruchala et al. recruited a small number of patients with newly diagnosed clinical hyperthyroidism or hypothyroidism (10 subjects in each group), and examined the associations between thyroid-axis hormones and irisin in their overall study group. Therefore, TSH levels ranged from 0 to 120 mIU/L (21). Conversely, this study recruited a larger number of previously thyroidectomized patients with a tighter TSH range (80 euthyroid and 16 subclinical hyperthyroid) and studied these associations in each subgroup and the overall cohort. In the same study, clinically hyperthyroid subjects had higher irisin levels compared with hypothyroid patients. However, the authors did not recruit a euthyroid group as was done in the present study, and therefore these results are not directly comparable. In a different study, Ates et al. recruited 37 newly diagnosed hypothyroid patients with Hashimoto's disease and 37 healthy controls, and showed that irisin was higher in the hypothyroid group compared with controls, especially in the obese population (36). The authors hypothesized that thyroid tissue-derived FNDC5/irisin (14) is released in the circulation as a result of the chronic inflammation of the thyroid. Therefore, in order to exclude this possibility, only thyroidectomized patients were recruited in the current study. On the other hand, in a very recent study aiming to explore whether the effect of thyroid hormones on serum irisin is time-dependent, circulating irisin levels were compared between hypothyroid patients with long-lasting autoimmune thyroiditis, thyroidectomized subjects with thyroid cancer who were withdrawn from their levothyroxine treatment for a four-week period, and healthy controls. It was found that irisin was lower in the long-lasting autoimmune thyroiditis group compared with the other two groups, and thus the authors hypothesized that prolonged hypothyroid myopathy might have a negative impact on irisin secretion (37). However, CK levels were not different between these groups, and therefore this conclusion remains uncertain. In addition, this study has the same inherent limitations and observational nature as the previous studies, as it only compares serum irisin levels between patients of different groups and does not study its intra-individual variation secondary to interventions as in the study presented here.

During follow-up of patients who underwent thyroidectomy due to thyroid cancer, it is necessary to evaluate the course of the disease with thyroglobulin measurements and/or whole-body scans. The latter can be performed with withdrawal of thyroxine treatment, resulting in a transient state of hypothyroidism (38). This standardized method was therefore used in the present study to elucidate the effect of experimentally altered levels of all thyroid-axis hormones and subsequent iatrogenic hypothyroidism on serum irisin levels. Thyroid hormone withdrawal did not result in any change in irisin levels. However, CK levels were significantly increased in these patients after becoming hypothyroid. Interestingly, although irisin has been associated with CK levels in experimentally hypothyroid and hyperthyroid rat models (22), as well as in a small group of clinical hyperthyroid and hypothyroid patients (21), no associations were found between irisin and CK levels in the euthyroid or iatrogenic hypothyroid state. Thus, it seems that irisin and CK are regulated in an independent manner, and it is unlikely that irisin might be used as a marker of muscle damage induced by an altered thyroid hormone status.

Additionally, it was hypothesized that TSH might exert extrathyroidal actions on muscle cells in a direct way. TSH receptors have been found in numerous tissues, including the liver (39), adipose tissue (40), and bone (41). However, there is a scarcity of data regarding TSH receptors in human skeletal muscle cells, with receptors found mostly in extraocular muscles (42) and cardiomyocytes (43), as well as C2C12 mouse skeletal muscle cells (44). In addition, although muscle-containing organs are the primary sites of irisin secretion (14), it was shown that irisin is expressed in other organs and mainly in adipose tissue, especially in states of increasing obesity (10,45).

Therefore, in order to elucidate whether TSH exerts a direct effect on irisin levels that is not counterbalanced by other members of the thyroid-axis hormone family, patients were further recruited and underwent a standardized protocol of TSH stimulation (rhTSH stimulation test) that selectively alters TSH levels (25,46) without affecting peripheral thyroid hormones as patients continued their treatment with levothyroxine. However, in order to avoid any possible effect of radioactive treatment and intramuscular injection on serum irisin, the patients did not receive radioactive treatment, and blood samples were collected at least 24 hours (day 3) after the intramuscular injection, while patients were followed for a total of 10 days.

Side effects after rhTSH stimulation protocol include nausea, vomiting, and flu-like symptoms, such as headache, fever, chills, and myalgia (46). However, none of the patients complained about any these symptoms, and it is unlikely that these symptoms might have had an impact on irisin levels. With this protocol, no significant change in the irisin or CK levels was found. As there are no published data on the possible effect of radioactive iodine on irisin levels, this should be examined in future research by specifically designed studies.

Finally, it has been proposed that irisin may be regulated in a sex-dimorphic manner (12,15). Therefore, the interaction between sex and irisin was analyzed in all the present studies. However, sex did not affect the results in any of the sub-studies.

The strengths of this study are its novel findings and its multifaceted design, with both cross-sectional and interventional human studies. To the best of the authors' knowledge, this is the first study to examine changes in serum irisin in models of an experimentally altered thyroid state in humans used to induce changes in all thyroid-axis hormones and TSH alone, thereby allowing cause–effect relationships to be established. In the interventional studies, patients with a previous history of thyroidectomy who were followed for recurrence of thyroid cancer were recruited. This allowed their thyroid status to be altered under standardized protocols that comply with bioethical standards. On the other hand, the relatively small sample size and the specific design of these studies should be considered when interpreting or generalizing the results in populations with different characteristics. In addition, since only short-term changes of thyroid state were studied, the possibility of significant effects of longer and/or more extreme changes of thyroid hormone levels on circulating irisin cannot be excluded.

Therefore, further studies are needed to investigate the effects of clinical and/or long-term changes of thyroid state on the levels of irisin. Inclusion of patients with clinical hyperthyroidism and clinical hypothyroidism and measurements of irisin levels at baseline and after treatment would further elucidate the downstream effect of thyroid-axis hormones on irisin levels. It is also possible that rhTSH may cross-react at high concentrations with irisin assays, and this remains to be fully elucidated.

In conclusion, the present data provide evidence that subclinical or interventional changes of thyroid state do not affect the levels of irisin in humans. The effect of clinical or long-term or more extreme changes of thyroid state on irisin levels should be further evaluated.

Footnotes

Author Disclosure Statement

The authors have nothing to disclose.

References

Poekes

, Lanthier

, Leclercq

. 2015. Brown adipose tissue: a potential target in the fight against obesity and the metabolic syndrome. Clin Sci, 129:933–949.

Vaitkus

, Farrar

, Celi

. 2015. Thyroid hormone mediated modulation of energy expenditure. Int J Mol Sci, 16:16158–16175.

Silva

. 1995. Thyroid hormone control of thermogenesis and energy balance. Thyroid, 5:481–492.

Lin

, Martagon

, Cimini

, Gonzalez

, Tinkey

, Biter

, Baxter

, Webb

, Gustafsson

, Hartig

, Phillips

. 2015. Pharmacological activation of thyroid hormone receptors elicits a functional conversion of White to Brown Fat. Cell Rep, 13:1528–1537.

Bostrom

, Wu

, Jedrychowski

, Korde

, Ye

, Lo

, Rasbach

, Bostrom

, Choi

, Long

, Kajimura

, Zingaretti

, Vind

, Tu

, Cinti

, Hojlund

, Gygi

, Spiegelman

. 2012. A PGC1-alpha-dependent myokine that drives brown-fat-like development of white fat and thermogenesis. Nature, 481:463–468.

Chen

, Huang

, Gusdon

, Qu

. 2015. Irisin: a new molecular marker and target in metabolic disorder. Lipids Health Dis, 14:2.

Huh

, Mougios

, Kabasakalis

, Fatouros

, Siopi

, Douroudos

, Filippaios

, Panagiotou

, Park

, Mantzoros

. 2014. Exercise-induced irisin secretion is independent of age or fitness level and increased irisin may directly modulate muscle metabolism through AMPK activation. J Clin Endocrinol Metab, 99:E2154–2161.

Qiu

, Cai

, Sun

, Schumann

, Zugel

, Steinacker

. 2015. Chronic exercise training and circulating irisin in adults: a meta-analysis. Sports Med, 45:1577–1588.

Peter

, Park

, Huh

, Wedick

, Mantzoros

. 2014. Circulating irisin levels are not affected by coffee intake: a randomized controlled trial. PloS One, 9:e94463.

10.

Moreno-Navarrete

, Ortega

, Serrano

, Guerra

, Pardo

, Tinahones

, Ricart

, Fernandez-Real

. 2013. Irisin is expressed and produced by human muscle and adipose tissue in association with obesity and insulin resistance. J Clin Endocrinol Metab, 98:E769–778.

11.

Panagiotou

, Mu

, Na

, Mukamal

, Mantzoros

. 2014. Circulating irisin, omentin-1, and lipoprotein subparticles in adults at higher cardiovascular risk. Metab Clin Exp, 63:1265–1271.

12.

Anastasilakis

, Polyzos

, Saridakis

, Kynigopoulos

, Skouvaklidou

, Molyvas

, Vasiloglou

, Apostolou

, Karagiozoglou-Lampoudi

, Siopi

, Mougios

, Chatzistavridis

, Panagiotou

, Filippaios

, Delaroudis

, Mantzoros

. 2014. Circulating irisin in healthy, young individuals: day-night rhythm, effects of food intake and exercise, and associations with gender, physical activity, diet, and body composition. J Clin Endocrinol Metab, 99:3247–3255.

13.

Vamvini

, Aronis

, Panagiotou

, Huh

, Chamberland

, Brinkoetter

, Petrou

, Christophi

, Kales

, Christiani

, Mantzoros

. 2013. Irisin mRNA and circulating levels in relation to other myokines in healthy and morbidly obese humans. Eur J Endocrinol, 169:829–834.

14.

Huh

, Panagiotou

, Mougios

, Brinkoetter

, Vamvini

, Schneider

, Mantzoros

. 2012. FNDC5 and irisin in humans: I. Predictors of circulating concentrations in serum and plasma and II. mRNA expression and circulating concentrations in response to weight loss and exercise. Metab Clin Exp, 61:1725–1738.

15.

Al-Daghri

, Alkharfy

, Rahman

, Amer

, Vinodson

, Sabico

, Piya

, Harte

, McTernan

, Alokail

, Chrousos

. 2014. Irisin as a predictor of glucose metabolism in children: sexually dimorphic effects. Eur J Clin Invest, 44:119–124.

16.

Palacios-Gonzalez

, Vadillo-Ortega

, Polo-Oteyza

, Sanchez

, Ancira-Moreno

, Romero-Hidalgo

, Meraz

, Antuna-Puente

. 2015. Irisin levels before and after physical activity among school-age children with different BMI: a direct relation with leptin. Obesity, 23:729–732.

17.

Bluher

, Panagiotou

, Petroff

, Markert

, Wagner

, Klemm

, Filippaios

, Keller

, Mantzoros

. 2014. Effects of a 1-year exercise and lifestyle intervention on irisin, adipokines, and inflammatory markers in obese children. Obesity, 22:1701–1708.

18.

Reinehr

, Elfers

, Lass

, Roth

. 2015. Irisin and its relation to insulin resistance and puberty in obese children: a longitudinal analysis. J Clin Endocrinol Metab, 100:2123–2130.

19.

Gouni-Berthold

, Berthold

, Huh

, Berman

, Spenrath

, Krone

, Mantzoros

. 2013. Effects of lipid-lowering drugs on irisin in human subjects in vivo and in human skeletal muscle cells ex vivo . PloS One, 8:e72858.

20.

Stengel

, Hofmann

, Goebel-Stengel

, Elbelt

, Kobelt

, Klapp

. 2013. Circulating levels of irisin in patients with anorexia nervosa and different stages of obesity—correlation with body mass index. Peptides, 39:125–130.

21.

Ruchala

, Zybek

, Szczepanek-Parulska

. 2014. Serum irisin levels and thyroid function-newly discovered association. Peptides, 60:51–55.

22.

Samy

, Ismail

, Nassra

. 2015. Circulating irisin concentrations in rat models of thyroid dysfunction—effect of exercise. Metab Clin Exp, 64:804–813.

23.

Huh

, Mantzoros

. 2015. Irisin physiology, oxidative stress, and thyroid dysfunction: What next?. Metab Clin Exp, 64:765–767.

24.

Mitsiades

, Pazaitou-Panayiotou

, Aronis

, Moon

, Chamberland

, Liu

, Diakopoulos

, Kyttaris

, Panagiotou

, Mylvaganam

, Tseleni-Balafouta

, Mantzoros

. 2011. Circulating adiponectin is inversely associated with risk of thyroid cancer: in vivo and in vitro studies. J Clin Endocrinol Metab, 96:E2023–2028.

25.

Pacini

, Schlumberger

, Dralle

, Elisei

, Smit

, Wiersinga

, European Thyroid Cancer T. 2006. European consensus for the management of patients with differentiated thyroid carcinoma of the follicular epithelium. Eur J Endocrinol, 154:787–803.

26.

Haugen

, Pacini

, Reiners

, Schlumberger

, Ladenson

, Sherman

, Cooper

, Graham

, Braverman

, Skarulis

, Davies

, DeGroot

, Mazzaferri

, Daniels

, Ross

, Luster

, Samuels

, Becker

, Maxon

3rd , Cavalieri

, Spencer

, McEllin

, Weintraub

, Ridgway

. 1999. A comparison of recombinant human thyrotropin and thyroid hormone withdrawal for the detection of thyroid remnant or cancer. J Clin Endocrinol Metab, 84:3877–3885.

27.

Smanik

, Ryu

, Theil

, Mazzaferri

, Jhiang

. 1997. Expression, exon–intron organization, and chromosome mapping of the human sodium iodide symporter. Endocrinology, 138:3555–3558.

28.

Loffler

, Muller

, Scheuermann

, Friebe

, Gesing

, Bielitz

, Erbs

, Landgraf

, Wagner

, Kiess

, Korner

. 2015. Serum irisin levels are regulated by acute strenuous exercise. J Clin Endocrinol Metab, 100:1289–1299.

29.

Polyzos

, Mantzoros

. 2015. An update on the validity of irisin assays and the link between irisin and hepatic metabolism. Metab Clin Exp, 64:937–942.

30.

Salvatore

, Simonides

, Dentice

, Zavacki

, Larsen

. 2014. Thyroid hormones and skeletal muscle—new insights and potential implications. Nat Rev Endocrinol, 10:206–214.

31.

Visser

, Heemstra

, Swagemakers

, Ozgur

, Corssmit

, Burggraaf

, van Ijcken

, van der Spek

, Smit

, Visser

. 2009. Physiological thyroid hormone levels regulate numerous skeletal muscle transcripts. J Clin Endocrinol Metab, 94:3487–3496.

32.

Lahesmaa

, Orava

, Schalin-Jantti

, Soinio

, Hannukainen

, Noponen

, Kirjavainen

, Iida

, Kudomi

, Enerback

, Virtanen

, Nuutila

. 2014. Hyperthyroidism increases brown fat metabolism in humans. J Clin Endocrinol Metab, 99:E28–35.

33.

Lapa

, Maya

, Wagner

, Arias-Loza

, Werner

, Herrmann

, Higuchi

. 2015. Activation of brown adipose tissue in hypothyroidism. Ann Med, 47:538–545.

34.

Broeders

, Bouvy

, van Marken Lichtenbelt

. 2015. Endogenous ways to stimulate brown adipose tissue in humans. Ann Med, 47:123–132.

35.

Moon

, Mantzoros

. 2014. Regulation of cell proliferation and malignant potential by irisin in endometrial, colon, thyroid and esophageal cancer cell lines. Metab Clin Exp, 63:188–193.

36.

Ates

, Altay

, Topcuoglu

, Yilmaz

. 2016. Circulating levels of irisin is elevated in hypothyroidism, a case-control study. Arch Endocrinol Metabolism, 60:95–100.

37.

Zybek-Kocik

, Sawicka-Gutaj

, Wrotkowska

, Sowinski

, Ruchala

. 2016. Time-dependent irisin concentration changes in patients affected by overt hypothyroidism. Endokrynol Pol, 2016 Feb 17. [Epub ahead of print]; DOI: 10.5603/EP.a2016.0030.

38.

Dow

, Ferrell

, Anello

. 1997. Quality-of-life changes in patients with thyroid cancer after withdrawal of thyroid hormone therapy. Thyroid, 7:613–619.

39.

Zhang

, Tian

, Han

, Ma

, Wang

, Guo

, Gao

, Zhao

. 2009. Presence of thyrotropin receptor in hepatocytes: not a case of illegitimate transcription. J Cell Mol Med, 13:4636–4642.

40.

Sorisky

, Bell

, Gagnon

. 2000. TSH receptor in adipose cells. Horm Metab Res, 32:468–474.

41.

Karga

, Papaioannou

, Polymeris

, Papamichael

, Karpouza

, Samouilidou

, Papaioannou

. 2010. The effects of recombinant human TSH on bone turnover in patients after thyroidectomy. J Bone Miner Metab, 28:35–41.

42.

Busuttil

, Frauman

. 2001. Extrathyroidal manifestations of Graves' disease: the thyrotropin receptor is expressed in extraocular, but not cardiac, muscle tissues. J Clin Endocrinol Metab, 86:2315–2319.

43.

Sellitti

, Hill

, Doi

, Akamizu

, Czaja

, Tao

, Koshiyama

. 1997. Differential expression of thyrotropin receptor mRNA in the porcine heart. Thyroid, 7:641–646.

44.

Ohn

, Han

, Park do

, Park

. 2013. Expression of thyroid stimulating hormone receptor mRNA in mouse C2C12 skeletal muscle cells. Endocrinol Metab, 28:119–124.

45.

Roca-Rivada

, Castelao

, Senin

, Landrove

, Baltar

, Belen Crujeiras

, Seoane

, Casanueva

, Pardo

. 2013. FNDC5/irisin is not only a myokine but also an adipokine. PloS One, 8:e60563.

46.

Meier

, Braverman

, Ebner

, Veronikis

, Daniels

, Ross

, Deraska

, Davies

, Valentine

, DeGroot

, et al. 1994. Diagnostic use of recombinant human thyrotropin in patients with thyroid carcinoma (phase I/II study). J Clin Endocrinol Metab, 78:188–196.