Intrinsic bioactivity of thyrotropin in human serum is inversely correlated with thyroid hormone concentrations. Application of a new bioassay using the FRTL-5 rat thyroid cell strain.

We have developed a new bioassay for thyrotropin (TSH) in human serum to evaluate bioactivity in normal individuals and patients with different degrees of primary hypothyroidism. Unpurified TSH in serum showed no stimulation of cyclic AMP production in cultured FRTL-5 rat thyroid cells, but after immunopurification showed potent stimulatory activity. Immunoaffinity purification permitted up to 400-fold concentration of serum TSH, allowing bioactivity measurements even in certain normal sera. The limit of detection in the FRTL-5 bioassay was 10 microU of human TSH per 0.5 ml incubate, and half-maximal responses for standard human TSH was 102 +/- 26 (+/- SE) microU/0.5 ml. Immunoaffinity-purified serum TSH varied in bioactivity-to-immunoactivity (B/I) ratios from less than 0.25 to 1.21 among four euthyroid subjects and eight primary hypothyroid patients. An inverse correlation was found between B/I ratios of immunopurified basal TSH and the serum-free T4 (r = -0.7237, P less than 0.01), T4 (r = -0.6650, P less than 0.05), and T3 (r = -0.6382, P less than 0.05). B/I ratios of immunopurified TSH from three hypothyroid patients before and after acute stimulation by thyrotropin-releasing hormone showed no significant change, despite major changes in serum TSH. In summary, the present study shows an inverse relationship between the metabolic status of an individual and the intrinsic bioactivity of TSH.