Original ArticleVariations In Adequate Levothyroxine Replacement Therapy In Patients With Different Causes Of Hypothyroidism
Section snippets
INTRODUCTION
The usual goal of treatment in patients with primary hypothyroidism is a normal serum thyroid-stimulating hormone (thyrotropin) level. The goal in patients with central hypothyroidism is a serum free thyroxine index (FT4I)—that is, thyroxine (T4) value × normalized tri-iodothyronine (T3) resin uptake—in the upper half of the normal range 1., 2.. Patients with various causes of hypothyroidism have been reported to be satisfactorily treated with levothyroxine at a mean daily dose of 1.6 μg/kg of
Study Cohort
We studied 181 adults with documented hypothyroidism who were receiving full levothyroxine replacement therapy. Hypothyroidism was defined as a significantly increased thyrotropin level (> 6 μU/mL) in the patients with primary hypothyroidism. All the patients with central hypothyroidism had a low FT4I and clinical hypothyroidism. Full replacement dosage in patients with primary hypothyroidism was defined as the dosage of levothyroxine (Synthroid, Levoxyl, or Levothroid) required for the patient
RESULTS
The time course for levothyroxine replacement dose requirements in 43 patients with Graves’ hyperthyroidism after 131I treatment is shown in Figure 1. The dose requirements increased significantly over time (P < 0.001). The data conformed well to a rectangular hyperbola, with the dose requirement asymptotically approaching 1.5 μg/kg per day.
The replacement doses of levothyroxine for the various causes of hypothyroidism are shown in Figure 2 A. An overall comparison among the groups showed
DISCUSSION
We found that the levothyroxine replacement dose in patients with hypothyroidism after 131I therapy varies as a function of time elapsed from such treatment. This result is likely due to the effect of continued production of thyroid-stimulating antibodies (5). Stabilization of the replacement dosage occurs approximately 7 years after 131I therapy. On the basis of the relationship between the levothyroxine dosage and the duration of hypothyroidism after radioiodine treatment observed in our
CONCLUSION
The data in this study demonstrate that the levothyroxine replacement dose varies with the cause of the hypothyroidism. Our findings are consistent with the hypothesis that the levothyroxine replacement dose is inversely correlated with the functionality of thyroid tissue.
ACKNOWLEDGMENT
Portions of this manuscript were presented at the 10th International Congress of Endocrinology and the 70th Annual Meeting of the American Thyroid Association. We thank Dr. Robert Utiger for invaluable editorial assistance, Dr. William Chin for use of his computer system for preparation of the illustrations, and Maxine Ricciuti for expert secretarial assistance in the preparation of the manuscript.
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