LETTER
Levothyroxine Therapy
Yair Liel, MD
1 April 1994 | Volume 120 Issue 7 | Pages 619-620
TO THE EDITOR:
In the review by Mandel and colleagues [1] on levothyroxine therapy, activated charcoal was omitted from the list of conditions affecting levothyroxine requirements by interfering with intestinal absorption [2]. By contrast, evidence in favor of interference with levothyroxine pharmacokinetics by lovastatin is weak [3].
The suggestion to separate by at least 3 hours the administration of levothyroxine from administration of medications that may interfere with its intestinal absorption could be risky because thyroxine undergoes enterohepatic circulation [4]. In our experience, separating the administration of an aluminum hydroxide from that of levothyroxine does not prevent considerable increase in serum levels of thyroid-stimulating hormone (TSH) [5]. Administration of levothyroxine and cholestyramine produced a marked increase in serum TSH in another patient, despite a minimum 4-hour separation between times of ingestion. I strongly agree with the suggestion to monitor thyroid function tests 1 month after initiation (or cessation) of drugs that affect levothyroxine requirements.
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Author and Article Information
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Soroka Medical Center; Ben-Gurion University of the Negev; Beer Sheva, Israel
1. Mandel, SJ, Brent GA, Larsen PR. Levothyroxine therapy in patients with thyroid disease. Ann Intern Med. 1993; 119:492-502.[Abstract/Free Full Text]
2. Bergman F, Halvorsen P, van der Linden W. Increased excretion of thyroxine by feeding activated charcoal to Syrian hamsters. Acta Endocrinol. 1967; 56:521-4.
3. Gormley GJ, Tobert JA. Drug interaction between thyroxine and lovastatin. N Engl J Med. 1989; 321:1342.
4. Hays M. Thyroid hormone and the gut. Endocrine Res. 1988; 14:203-24.
5. Sperber AD, Liel Y. Evidence for interference with the intestinal absorptin of levothyroxine sodium by aluminum hydroxide. Arch Intern Med. 1992; 152:183-4.
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