Effects of small doses of bovine TSH on serum levels of free and total thyroid hormones, their degradation products, and diiodotyrosine

Abstract

Abstract. Bovine TSH was administered iv to 10 normal volunteers in doses of 2.5, 7.5, 15 and 30 mU/kg. Brisk elevations of serum diiodotyrosine occurred already after the smallest dose (mean, + 183%) while larger doses had only slight additional effects. T3 rose much higher than T4 (+71% compared to +23% after 15 mU bTSH/kg), and free thyroid hormones exhibited changes similar to total T3 and total T4. The mean absolute increase in serum fT3 ranged from 2.03 to 9.04 pmol/l and proved to be an easily measurable parameter for the TSH effect. Dose-response effects were seen for the increases of fT4, fT3 and T3. TBG and rT3 did not change but the degradation product 3,3'-T2 showed large increments of serum levels. There was no correlation between the response of T3 and T4, fT3 and fT4, or diiodotyrosine and any of the other parameters of thyroid function. The interindividual differences in the magnitude of thyroid hormone response to TSH were considerable, and there was no relationship between this response and thyroid volume by ultrasound. We conclude that direct stimulation of the thyroid gland with bTSH in small doses leads to consistent increases of thyroid hormones, especially T3 and fT3, that the response varies between individuals, and that the precursor diiodotyrosine is released together with thyroid hormones.