Studies on the control of thyrotrophin secretion in normal man.
The effect of graded doses of thyroxine (T4) and triiodothyronine (T3) separately on the pituitary-thyroid axis has been studied in normal subjects. It has been demonstrated that small doses (50 mug T4 or 20 mug T3) cause significant suppression of the pituitary-thyroid axis. There was  a strong negative correlation between the TSH value 20 min after TRH and the dose of thyroid hormone. T4 supplements led to a progressive dose-related rise in serum T4 and no significant change in T3 until high daily doses (greater than 200 mug) were administered. T3 supplements led to a progressive reduction in serum T4 and there was no change in T3 until substantial supplements were given (greater than 50 mug/day). Concurrent administration of 6 PTU had no effect on the pattern of pituitary suppression or thyroid hormone concentrations in contrast to its reported effect in hypothyroid subjects.
D000328:Adult; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D008297:Male; D010902:Pituitary Gland / Q000187:drug effects; D011441:Propylthiouracil / Q000494:pharmacology; D013972:Thyrotropin / Q000378:metabolism*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology; D013974:Thyroxine / Q000097:blood / Q000494:pharmacology*; D014284:Triiodothyronine / Q000097:blood / Q000494:pharmacology*
D014284:Triiodothyronine; D013973:Thyrotropin-Releasing Hormone; D011441:Propylthiouracil; D013972:Thyrotropin; D013974:Thyroxine