Clinical Chemistry
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Clinical Chemistry 21: 1388-1392, 1975;
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Clinical Chemistry, Vol 21, 1388-1392, Copyright © 1975 by the American Association for Clinical Chemistry

Thyroid-Function Tests in Diphenylhydantoin-Treated Patients

Melvin R. Stjernholm 1, Robert N. Alsever 1, and Merritt C. Rudolph 1

1 Division of Endocrinology, Department of Medicine, University of Colorado Medical Center; and Veterans Administration Hospital, Denver, Cob. 80220.

We compared the free-thyroxine index in normal adults and in euthyroid patients taking diphenylhydantoin. All subjects had normal serum thyrotropin concentrations. Serum thyroxine concentrations were determined by two commonly used competitive protein-binding assays, which yielded slightly different values, but which consistently showed the same degree of decrease in mean serum thyroxine concentration in drug-treated patients as compared to the normal subjects. When 14C-labeled diphenylhydantoin was added to serum before the assay, it was separated from thyroxine in the Ames method, whereas by the Murphy-Pattee method both drug and thyroxine were extracted together. Thus, the decrease in serum thyroxine concentrations during diphenylhydantoin therapy cannot be the result of drug interference with the binding of thyroxine to binding proteins in the assays. Triiodothyronine uptake, evaluated by two methods, was identical in the two groups. The free-thyroxine indexes for all normal persons were within the manufacturer's normal range, but 21% of the drug-treated patients had subnormal indexes by the Ames method; the indexes as measured by the Murphy— Pattee method were in the lower half of the normal range. Because the triiodothyronine uptake was unaffected by the drug treatment, the decreases in the indexes must have resulted from the lower serum thyroxine concentrations. We conclude that the free-thyroxine index may not provide a valid estimate of either the clinical status or the free-thyroxine concentration in patients taking diphenylhydantoin.


Key Words: thyroid status of diphenylhydantointreated patients • determinants of euthyroidism • "kit" methods

Submitted on January 18, 1975
Accepted on May 29, 1975




The following articles in journals at HighWire Press have cited this article:


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M. I. Surks and C. R. DeFesi
Normal Serum Free Thyroid Hormone Concentrations in Patients Treated With Phenytoin or Carbamazepine: A Paradox Resolved
JAMA, May 15, 1996; 275(19): 1495 - 1498.
[Abstract] [PDF]


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Copyright © 1975 by the American Association for Clinical Chemistry.