Correlations of free thyroid hormones measured by tandem mass spectrometry and immunoassay with thyroid-stimulating hormone across 4 patient populations.

BACKGROUND Accurate measurement of free thyroid hormones is important for managing thyroid disorders. Ultrafiltration liquid chromatography tandem mass spectrometry (LC-MS/MS) can reliably measure the concentrations of small molecules, including thyroid hormones. Our study was designed to compare free thyroid hormone measurements performed with immunoassay and LC-MS/MS. METHODS We studied the performance of LC-MS/MS in 4 different populations comprising pediatric patients, euthyroid adults, and healthy nonpregnant and pregnant women. The samples obtained from each population numbered 38, 200, 28, and 128, respectively. Free thyroxine, free triiodothyronine, and thyroid-stimulating hormone (TSH) concentrations were documented. RESULTS LC-MS/MS measurement of free thyroid hormones provided better correlation with log-transformed serum TSH in each population and also the populations combined. The correlations between free thyroxine measured by LC-MS/MS and log TSH in the pediatric outpatients and healthy adults were -0.90 and -0.77, respectively. The correlations for immunoassay were -0.82 and -0.48. The correlations between free triiodothyronine measured by LC-MS/MS and TSH for both pediatric and healthy adult populations were -0.72 and -0.68, respectively. CONCLUSIONS Free thyroid hormone concentrations measured by LC-MS/MS correlate to a greater degree with log TSH values compared to concentrations measured by immunoassay. This correlation was maintained across the patient populations we studied and may reflect the accuracy and specificity of LC-MS/MS. The superior ability of LC-MS/MS to enable documentation of the well-known thyroid hormone-TSH relationship supports the use of this measurement technique in a variety of clinical situations.