STUDIES ON THE CONTROL OF THYROTROPHIN SECRETION IN NORMAL MAN

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Abstract
The effect of graded doses of thyroxine (T4) and triiodothyronine (T3) separately on the pituitary-thyroid axis was studied in normal subjects. Small doses (50 .mu.g T4 or 20 .mu.g T3) significantly suppressed the pituitary-thyroid axis. There was a strong negative correlation between the TSH [thyrotropin] value 20 min after TRH [TSH-releasing hormone] 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 (> 200 .mu.g) were administered. T3 supplements led to a progressive reduction in serum T4, and there was no change in T3 until substantial supplements were given (> 50 .mu.g/day). Concurrent administration of 6 PTU [propylthiouracil] had no effect on the pattern of pituitary suppression or thyroid hormone concentrations in contrast to its reported effect in hypothyroid subjects.