Complex relationship between free thyroxine and TSH in the regulation of thyroid function.

Objective: The present study re-evaluates the inverse log TSH-free thyroxine (fT(4)) relationship, which has generally been assumed to characterize the thyroid pituitary hypothalamic feedback regulation in thyroid function.

Design And Methods: The correlation between fT(4) and TSH was analyzed in two data sets from differing time periods involving 3223 and 6605 patients referred for thyroid testing, representing the whole range of thyroid functions from hypothyroidism to hyperthyroidism.

Results: We found that the data do not support a linear log TSH-fT(4) relationship; instead, the correlation's gradient varies with thyroid function.