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Transient Elevation of Serum Thyroid Hormone Concentration After Initiation of Replacement Therapy in Myxedema

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▸Requests for reprints should be addressed to Samuel Refetoff, M.D.; Department of Medicine, University of Chicago, 950 East 59th Street; Chicago, IL 60637.

Chicago, Illinois

© 1980 American College of PhysiciansAmerican College of Physicians

Ann Intern Med. 1980;92(4):491-495. doi:10.7326/0003-4819-92-4-491
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Measurements of thyroid hormone concentrations in serum are commonly used to determine the proper dose of hormone replacement. We have noted that early in the course of thyroxine (T4) replacement in myxedema, serum T4 concentrations may be transiently elevated before reaching a lower "steady-state" level. This observation is illustrated in a study of six patients. Serum T4, free thyroxine index, and triiodothyronine (T3) rose to peak concentrations at 2 to 6 weeks, 35% to 120% above the values achieved 4 to 8 months later. Values were transiently in the thyrotoxic range in five of the six patients. This phenomenon is most likely due to a decrease in the metabolic clearance rate of the absorbed hormone associated with hypometabolism. Thus, serum T4 and T3 concentrations during the first 6 months of therapy do not reflect the optimal dose of T4 replacement on a long-term basis.





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