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.