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Thyroxin Absorption Test

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Question

In patients with suspected T-4 malabsorption, is there a “standardized” T-4 absorption test during which time patients are given their T-4 dose supervised in the office, and have their labs monitored at intervals? I have done this in the past, and have derived useful information; however, I am uncertain as to the best time to draw samples following the dose, and exactly what tests will provide the most information. i.e. to order TSH, T-4, and/or free T-4 levels, at what intervals for how long?

Thanks,

Victor E. Silverman, M.D.

Response

The problem to test T4 absorption using ‘cold’-, i.e. non-radioactive T4 is the fact that T4 disappearance from blood is relatively slow (10%/day). This means that the peak of T4 after absorption does not give any quantitative information. The best way to measure T4 absorption is to administer the patient intravenously a known amount of 131I labeled T4 and simultaneously differently labeled i.e. 125I T4 orally. The differences in the serum kinetics of the 2 labels shows the fraction that is absorbed from the intestines. Regards Georg Hennemann, MD, PhD, FRCP, FRCPE Note added—The FDA and others use a standard “AREA under the curve” measurement with 100ug tablets, but it is fairly insensitive.. The test can be done with a 1000ug dose, safely, but I could not find an available reference to this in my quick search in MEDLINE.

L De Groot, MD

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