A Thyroid Suppression Test Using a Single Dose of L-Thyroxine

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Abstract
Thyroid suppression can be tested with a single dose of L-thyroxine. Three milligrams of thyroxine was administered orally after a base-line 24-hour radioactive iodine uptake test; 24-hour uptake seven days later was taken as the post-suppression value. In all normal subjects the level was suppressed to less than 50 per cent of the base-line value, whereas no patient with Graves's disease achieved 50 per cent suppression. There was no detectable metabolic impact from this L-thyroxine dose, except for a mild lowering of the serum cholesterol. Individual subjects, undergoing consecutive tri-iodothyronine and thyroxine suppression tests, demonstrated a similar degree of suppression with either test. Because the conventional tri-iodothyronine suppression test requires daily doses of tri-iodothyronine, erratic self-administration may lead to invalid test results. Since the single-dose L-thyroxine suppression test apparently eliminates this potential source of error, it may be considered the preferred thyroid suppression test to be performed on an outpatient basis.

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