Elsevier

Endocrine Practice

Volume 5, Issue 5, September–October 1999, Pages 233-238
Endocrine Practice

Original Article
Variations In Adequate Levothyroxine Replacement Therapy In Patients With Different Causes Of Hypothyroidism

https://doi.org/10.4158/EP.5.5.233Get rights and content

ABSTRACT

Objective

To compare the levothyroxine replacement dose in 181 patients with various causes of hypothyroidism.

Methods

We analyzed the dose of levothyroxine used in the following five patient groups: (1) 37 patients with hypothyroidism after radioiodine therapy for Graves’ thyrotoxicosis who were receiving a stable (for at least 4 years) replacement dose (mean time after 131I therapy, 11.3 years); (2) 36 patients with Hashimoto’s hypothyroidism (chronic autoimmune thyroiditis with a goiter or positive test results for antithyroid antibodies); (3) 36 patients with central hypothyroidism; (4) 36 patients with hypothyroidism after near-total thyroidectomy and 131I therapy for thyroid carcinoma with negative total-body 131I scans who were euthyroid when receiving levothyroxine; and (5) 36 patients with atrophic thyroiditis (no goiter and negative test results for antithyroid antibodies). Adequacy of levothyroxine replacement dose was defined as a normal thyrotropin level and clinical euthyroidism in patients with primary hypothyroidism and a serum free thyroxine index in the upper half of the normal range in conjunction with clinical euthyroidism in patients with central hypothyroidism.

Results

The mean (± standard error of the mean) replacement dosage of levothyroxine (μg/kg per day) in patients with atrophic thyroiditis (1.26 ± 0.07) was lower (P < 0.05) than in patients with Hashimoto’s hypothyroidism (1.59 ± 0.07) and those with hypothyroidism after radioiodine therapy (1.56 ± 0.05). These doses, in turn, were lower (P < 0.01) than those in patients with central hypothyroidism (1.88 ± 0.10) or euthyroid thyroid carcinoma (2.08 ± 0.07). In a separate analysis, the levothyroxine dose in 43 patients with hypothyroidism after 131I treatment was evaluated serially over time. The mean levothyroxine dosage increased from 0.87 ± 0.12 at 6 months after 131I therapy to 1.57 ± 0.09 at 7 years (P < 0.001). The serum thyrotropin concentration (in μU/mL) during levothyroxine therapy in patients with central hypothyroidism (0.31 ± 0.08) was lower (P < 0.01) than in patients with hypothyroidism after 131I therapy (1.69 ± 0.37), Hashimoto’s hypothyroidism (1.39 ± 0.20), atrophic thyroiditis (1.86 ± 0.22), and euthyroid thyroid carcinoma (1.48 ± 0.26).

Conclusion

The levothyroxine replacement dose varies with the cause of the hypothyroidism. (Endocr Pract. 1999;5:233-238).

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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