IMPROVED SENSITIVITY OF SERUM THYROTROPIN MEASUREMENTS - STUDIES ON SERUM SEX HORMONE-BINDING GLOBULIN IN PATIENTS WITH REDUCED SERUM THYROTROPIN

Most serum TSH assays have a working sensitivity (i.e. the lowest TSH value with an inter-assay coefficient of variation below 10%) around 0.15-0.4 mU/l, which also is the critical area for cut off for further thyroid profile testing when serum TSH is used as a 'first line test'. A new assay (BeriLux hTSH) based on chemiluminescence was evaluated, and demonstrated a theoretical sensitivity (mean + 2 SD of the zero standard) of 0.005 mU/l and a working sensitivity as low as 0.04 mU/l. Reference range was 0.18-2.60 mU/l (N = 33). Sixty-eight percent (13/19) of hyperthyroid patients had serum TSH <0.005 mU/l, all had serum TSH <0.037 mU/l. We studied two groups of patients with normal free T4 and T3 indices but serum TSH <0.15 mU/l as measured by an immunoradiometric assay. Thirty-five percent (7/20) of patients with nontoxic goitre and 5% (1/20) of L-T4 treated patients had serum TSH <0.005 mU/l; and in 60% (11/20) and 30% (6/20), respectively, the levels overlapped the total range for hyperthyroidism. Serum levels of sex hormone-binding globulin expressed as percent of the reference median for the relevant sex (SHBG%) were elevated in both groups of patients (p < 0.01). Approximately 50% of those with serum TSH overlapping the hyperthyroid range had serum SHBG% levels above reference range. In conclusion, this assay seems superior to most previously reported assays concerning working sensitivity, but it still leaves us with a group of clinically euthyroid patients who has unmeasurably low serum TSH. Many of these patients have incresed serum SHBG%, and might in fact be subclinically hyperthyroid.