Tandem mass spectrometry as a novel tool for elucidating pituitary-thyroid relationships

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 (T(4)), triiodothyronine (T(3)), free T(4) (FT(4)), and TSH levels were documented on two occasions before thyroidectomy. After thyroidectomy, patients were treated with levothyroxine (LT(4)) to achieve either a normal or low serum TSH concentration. All laboratory evaluations were repeated twice while patients were taking LT(4). 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: FT(4) and total T(3) correlated better with the log-transformed TSH when measured by LC-MS/MS. Postthyroidectomy the closest correlation was between log TSH and FT(4) (r = 0.86, p < 0.001). The next best correlation was between log TSH and total T(3) (r = 0.71, p < 0.001). When all data points were combined, the slope of the relationship between log TSH and total T(3) was relatively blunted compared with the log TSH-FT(4) slope (slope - 0.39 vs. - 1.38; p < 0.001), perhaps suggesting autoregulation of T(3) 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 FT(4) and total T(3) measured by tandem mass spectrometry correlate well with TSH. However, T(3) correlates slightly less well and has a relatively blunted relationship with the log-transformed TSH. These paired data suggest that in LT(4)-replaced patients T(3) concentrations are held stable in the face of fluctuating T(4) concentrations.

Figures

FIG. 1.

(A, B) Free thyroxine (FT4) measured by immunoassay and liquid chromatography–tandem mass spectrometry (LC-MS/MS), respectively, plotted against log thyroid-stimulating hormone (TSH). Filled squares represent prethyroidectomy data points, benign disease; open squares represent prethyroidectomy data points, thyroid cancer. Filled triangles represent postthyroidectomy data points, benign disease; open triangles represent postthyroidectomy data points, thyroid cancer. (C) Agreement between FT4 measured by immunoassay versus tandem mass spectrometry. Filled squares represent prethyroidectomy data points; circles represent postthyroidectomy data points.

FIG. 1.

(A, B) Free thyroxine (FT4) measured by immunoassay and liquid chromatography–tandem mass spectrometry (LC-MS/MS), respectively, plotted against log thyroid-stimulating hormone (TSH). Filled squares represent prethyroidectomy data points, benign disease; open squares represent prethyroidectomy data points, thyroid cancer. Filled triangles represent postthyroidectomy data points, benign disease; open triangles represent postthyroidectomy data points, thyroid cancer. (C) Agreement between FT4 measured by immunoassay versus tandem mass spectrometry. Filled squares represent prethyroidectomy data points; circles represent postthyroidectomy data points.

FIG. 2.

(A, B) Triiodothyronine (T3) measured by immunoassay and liquid chromatography–tandem mass spectrometry (LC-MS/MS), respectively, plotted against log thyroid-stimulating hormone (TSH). Filled squares represent prethyroidectomy data points, benign disease; open squares represent prethyroidectomy data points, thyroid cancer. Filled triangles represent postthyroidectomy data points, benign disease; open triangles represent postthyroidectomy data points, thyroid cancer. (C) Agreement between T3 measured by immunoassay versus tandem mass spectrometry. Filled squares represent prethyroidectomy data points; circles represent postthyroidectomy data points.

FIG. 2.

(A, B) Triiodothyronine (T3) measured by immunoassay and liquid chromatography–tandem mass spectrometry (LC-MS/MS), respectively, plotted against log thyroid-stimulating hormone (TSH). Filled squares represent prethyroidectomy data points, benign disease; open squares represent prethyroidectomy data points, thyroid cancer. Filled triangles represent postthyroidectomy data points, benign disease; open triangles represent postthyroidectomy data points, thyroid cancer. (C) Agreement between T3 measured by immunoassay versus tandem mass spectrometry. Filled squares represent prethyroidectomy data points; circles represent postthyroidectomy data points.

FIG. 3.

(A) Free thyroxine (FT4) levels plotted against log thyroid-stimulating hormone (TSH). Each line is an individual patient with pre- and postthyroidectomy data points all on the same line. (B) Triiodothyronine (T3) levels plotted against log TSH. Each line is an individual patient with pre- and postthyroidectomy data points all on the same line.