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Dynamics of thyroid hormone suppression of serum thyrotropin: an invited commentary

The insensitivity of the early TSH radioimmunoassays limited investigations of thyroid hormone action on pituitary TSH secretion to studies of hypothyroid rats and humans (1, 2). Now that more sensitive TSH immunometric assays are able to measure serum TSH concentrations in the subnormal range (3), it is possible to study thyroid hormone inhibition of pituitary TSH secretion in euthyroid human subjects. In the welldesigned study of Sydow et al. (4), groups of euthyroid male volunteers were given different oral doses of thyroxine (25–150 µg q.i.d.) for 10 days, during which time the relationships between the thyroxine dose and the resultant suppression of serum TSH and thyroglobulin (TG) concentrations were studied. The data showed that both serum TSH and Tg concentrations fell over time with single monoexpontential suppression patterns. Further, the rates of decline for both serum TSH and serum Tg were proportional to the thyroxine dose administered. It should be emphasized