Abstract
Background:
Our objective was to determine the performance of liquid chromatography–tandem mass spectrometry (LC-MS/MS) in documenting both group and individual relationships between thyroid hormone and thyroid-stimulating hormone (TSH) concentrations.
Methods:
This was a prospective analysis of 50 euthyroid patients undergoing thyroidectomy. Thyroxine (T4), triiodothyronine (T3), free T4 (FT4), and TSH levels were documented on two occasions before thyroidectomy. After thyroidectomy, patients were treated with levothyroxine (LT4) to achieve either a normal or low serum TSH concentration. All laboratory evaluations were repeated twice while patients were taking LT4. Thyroid hormone concentrations were documented by both immunoassay and LC-MS/MS, and their relationship with TSH was studied both in the entire group and in individual patients pre- and postthyroidectomy.
Results:
FT4 and total T3 correlated better with the log-transformed TSH when measured by LC-MS/MS. Postthyroidectomy the closest correlation was between log TSH and FT4 (r = 0.86, p < 0.001). The next best correlation was between log TSH and total T3 (r = 0.71, p < 0.001). When all data points were combined, the slope of the relationship between log TSH and total T3 was relatively blunted compared with the log TSH–FT4 slope (slope − 0.39 vs. − 1.38; p < 0.001), perhaps suggesting autoregulation of T3 in response to the altered conditions postthyroidectomy.
Conclusion:
LC-MS/MS is an excellent tool for documenting the known physiological phenomenon of a log–linear relationship between TSH and thyroid hormone concentrations. In a group of patients studied pre- and postthyroidectomy, both FT4 and total T3 measured by tandem mass spectrometry correlate well with TSH. However, T3 correlates slightly less well and has a relatively blunted relationship with the log-transformed TSH. These paired data suggest that in LT4-replaced patients T3 concentrations are held stable in the face of fluctuating T4 concentrations.