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Frank M. Grund, MD; Catherine B. Niewoehner, MD

Arch Intern Med. 1989;149(4):921-924.

Abstract
• Eleven patients, with a history of hypothyroidism, who had hyperthyroxinemia and an elevated free thyroxine index but normal serum triiodothyronine concentrations on levothyroxine replacement underwent levothyroxine dose reduction at three-month intervals until the free thyroxine index fell into the normal range. All were clinically euthyroid throughout. Normalization of the thyrotropin response to thyrotropinreleasing hormone occurred concomitantly, indicating correction of subtle hyperthyroidism. The mean thyroxine dose decreased from 161 µg/d (2.06 µg/kg) to 120 µg/d (1.51 µg/kg). The resting heart rate fell in eight of 11 patients (P<.02). The left ventricular ejection fraction decreased in eight of 11 patients, although the decrease was not statistically significant. Considering the sensitivity of pituitary, cardiac, and bone tissue to even a small excess of thyroxine over time, hyperthyroxinemia associated with an elevated free thyroxine index should be corrected even in patients taking levothyroxine replacement who are clinically euthyroid and whose serum triiodothyronine concentrations are within normal limits.

(Arch Intern Med 1989;149:921-924)

Author Affiliations
From the Department of Nuclear Medicine (Dr Grund) and Endocrine Metabolic Section (Dr Niewoehner), Veterans Administration Medical Center, and the Departments of Medicine (Drs Grund and Niewoehner) and Radiology (Dr Grund), University of Minnesota, Minneapolis.

Footnotes
Accepted for publication Oct 26, 1988.

Reprint requests to Department of Nuclear Medicine, Veterans Administration Medical Center, 54th Street and 48th Avenue South, Minneapolis, MN 55417 (Dr Grund).

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