Evidence for Thyroid Hormone as a Positive Regulator of Serum Thyrotropin Bioactivity

Context: The regulation of TSH bioactivity in humans is not completely understood. Objective: The aim of the study was to investigate the role of serum thyroid hormones in regulating the bioactivity of TSH. Design: We determined in vitro TSH bioactivity and glycosylation in nine patients (six females and three males, age 41.3 yr) with primary hypothyroidism before and after l-T4 replacement, in 11 age- and sex-comparable controls (seven females and four males, age 37.6 yr), and in two thyroidectomized patients with TSH-secreting adenomas during and after l-T4 withdrawal. Methods: In vitro TSH bioactivity was measured by a sensitive and specific bioassay based on cAMP generation by Chinese hamster ovary cells transfected with human TSH receptor. TSH glycosylation was assessed by concanavalin A lectin and ricin column affinity chromatography. Results: In vitro TSH bioactivity in hypothyroid patients was low as compared with controls (0.48 ± 0.1 vs. 1.1 ± 0.2; P = 0.004) and increased during l-T4 (0.48 ± 0.1...