Re: “Seasonal Changes in Serum Thyrotropin Concentrations Observed from Big Data Obtained During Six Consecutive Years from 2010 to 2015 at a Single Hospital in Japan” by Yoshihara et al. ( Thyroid 2018;28:429–436)

Dear Editor:

The data presented by Yoshihara et al. (1) document a circannual thyrotropin (TSH) variation in a very large Japanese data set. The authors report a correlation between mean ambient temperature and TSH, suggesting that the seasonal variation in TSH represents a physiological response to variations in ambient temperature, and call attention to the effects of secular factors, such as clothing and heating, on thermoregulation.

In addition to environmental temperature, another possible driver of the observed seasonal variation in TSH is dietary iodine. Iodine intake in the Japanese is among the highest in the world due to the widespread consumption of seaweed (2), and seasonal variation in the iodine content of seaweed has been documented (3). Among populations in whom diary products represent a significant source of dietary iodine, seasonal variation in dairy product iodine content corresponds to circannual variations in human iodine excretion (4). Seasonal variation in iodine consumption can affect thyroid hormone secretion and may contribute to the observed seasonal rhythm in TSH (5).