Integration of thyroid hormones with hypothalamic factors on pituitary TSH secretion

A study employing a newly developed fourth generation immunometric serum TSH assay (assay limit 0.001 mu/l was performed to determine the temporal pattern of inhibition of serum TSH in response to a maximal suppressive dose of thyroid hormone (T3, T4 and TRIAC) as compared to the pattern produced by dopamine (DA) and dexamethasone (DEX). Mean onset of inhibition was 24, 48 and 68 minutes for thyroid hormone, DA and DEX, respectively. The inhibitory pattern was uniform and reproducible for the same or different individuals if data were normalized respective to basal TSH values. Inhibitory pattern with thyroid hormone formed two distinct log linear functions: A rapid phase (phase 1) spanning 48 minutes to 24 hours and a slower phase (phase 2) bridging 24 to 1,000 hours. A diurnal variation in phase 1, but not in phase 2 suppression, was observed. A phase 1 response was also produced by DA and DEX but not phase 2. Low dose TRH infusion studies indicated that thyroid hormone and DEX inhibited thyrotroph response at or beyond the TRH receptor, while DEX appeared to inhibit endogenous TRH secretion. No additive effects of DA or DEX on T3 inhibition were seen in phase 1. These studies provide new insight into the mechanism by which these endocrine factors complementarily regulate TSH secretion in man.