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Changes in Thyroid Hormone Levels during Growth Hormone Therapy in Initially Euthyroid Patients: Lack of Need for Thyroxine Supplementation¹

Department of Pediatrics, Medical College of Wisconsin (D.T.W.), Milwaukee, Wisconsin 53226; and the Department of Medical Affairs, Genentech, Inc. (N.G., B.S.), South San Francisco, California 94080

Address all correspondence and requests for reprints to: David T. Wyatt, M.D., Medical College of Wisconsin, Department of Pediatrics, 8701 Watertown Plank Road, Milwaukee, Wisconsin 53226. E-mail: dtwyatt{at}mcw.edu

The occurrence of central hypothyroidism in previously euthyroid children during GH therapy has been reported with widely varying incidence. We monitored the acute effects on the hypothalamic-pituitary-thyroid axis in 15 euthyroid children with classic GH deficiency during the first year of GH therapy. All were initially euthyroid, as assessed by normal baseline TSH, T₄, free T₄, and T₃ levels and negative antithyroid antibodies. A thyroid profile (T₄, free T₄ index, T₃, rT₃, and TSH) was performed at baseline and 1, 3, 6, 9, and 12–15 months after GH therapy began; a TRH stimulation test was performed at baseline and after 1, 3, and 9 months of therapy. By 1 month, there were significant decreases in T₄, free T₄ index, and rT₃, and significant increases in T₃ and the T₃/T₄ ratio. The changes from baseline values were greatest at 1 month, were almost universal for all thyroid values, and showed a gradual return to baseline from 3–12 months. There were no clinical signs of hypothyroidism and no change in baseline or TRH-stimulated TSH levels or in cholesterol levels, and all patients grew at velocities expected for the treatment schedule. There is little evidence for the development of clinically significant hypothyroidism in the great majority of initially euthyroid patients after GH therapy is begun. T₄ supplementation is seldom needed in such patients.

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