European Journal of Clinical Pharmacology

, Volume 26, Issue 5, pp 633–634

Measurement of free thyroid hormones in patients on long-term phenytoin therapy

  • J. A. Franklyn
  • M. C. Sheppard
  • D. B. Ramsden
Short Communications

DOI: 10.1007/BF00543499

Cite this article as:
Franklyn, J.A., Sheppard, M.C. & Ramsden, D.B. Eur J Clin Pharmacol (1984) 26: 633. doi:10.1007/BF00543499

Summary

Total thyroxine and triiodothyronine levels are often reduced in long-term phenytoin therapy, a finding at variance with the euthyroid clinical state of these subjects. We have measured free (biologically active) thyroid hormone levels in 31 patients on phenytoin therapy and have found a significant reduction in measurements of both free thyroxine and free triiodothyronine. In contrast, addition of phenytoin to normal serum in vitro resulted in an increase in free thyroxine and free triiodothyronine concentrations. The findings in vitro are consistent with displacement of thyroid hormones from thyroxine binding globulin while the reduced free hormone levels in vivo confirm that the effects of phenytoin are not confined to binding inhibition and suggest that phenytoin has induced a change in the cellular metabolism of thyroid hormones.

Key words

phenytoin thyroid hormones thyroid function tests free thyroxine free triiodothyronine 

Copyright information

© Springer-Verlag 1984

Authors and Affiliations

  • J. A. Franklyn
    • 1
  • M. C. Sheppard
    • 1
  • D. B. Ramsden
    • 1
  1. 1.Department of MedicineUniversity of Birmingham, Queen Elizabeth HospitalBirminghamEngland