Triiodothyronine Withdrawal - A Possible Test for Hypothalamic-Pituitary-Thyroid Adequacy

Abstract

The aim of this study was to develop a test which could be used as a complement to the assay of serum TSH and the TRH test in the evaluation of hypothalamic-pituitary-thyroid adequacy.

When T₃, 75 µg per day, was given to healthy subjects for 7 days (days 1-7), a significant rebound of serum TSH over the basal value was found on at least two of days 15-17 in nine of ten cases, provided potassium iodide, 10 mg per day, was also given on days 6-16. A much more pronounced rebound of TSH was obtained when thyroid hormone release was more rigorously blocked with the following medication: 125 µg T₃ on days 1-7, 500 mg KI on days 1-16, and 400 mg of a slowly resorbed lithium carbonate preparation on days 1-16. In the latter case TSH rose from an average basal value of 0.96 µU/ml to a mean of 3.41 µU/ml on day 17 in 10 healthy subjects. In a subsequent experiment a similar TSH rebound was obtained with 125 µg T₃ (days 1-7) and 500 mg KI (days 1-16) only.

The mechanism whereby the TSH rebound is brought about, remains to be established. One possibility is a transshypothalamic action. In this case the procedure may prove useful as a clinical test for the detection of slight degrees of hypothalamic insufficiency which cannot be diagnosed with certainty by the TRH test.

The present experiments provide indirect evidence for a suppressive action of comparatively small doses (10 mg per day) of KI on thyroid hormone release in healthy subjects.

Key words

Hypothalamic - Pituitary - Thyroid Adequacy - Thyrotropin - Iodide - Triiodothyronine - Thyroxine