Difference in Pituitary-Thyroid Feedback Regulation in Hypothyroid Patients, Depending on the Severity of Hypothyroidism

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Abstract
In an attempt to study pituitary-thyroid interplay during replacement therapy for hypothyroidism, T4 (75–150 μg/day) was administered for at least 3 months. A small dose of T4 (75 μg/day) significantly depressed basal and TRH-stimulated TSH levels in normal subjects without significantly elevating serum T4 and T3 concentrations. In patients with severe hypothyroidism and marked enlargement of the sella turcica, T4 (2.53 μg/kg BW) normalized the serum T4 and slightly elevated the serum T3 but failed to normalize basal and TRH-stimulated TSH levels. In patients with moderate hypothyroidism and moderate enlargement of the sella turcica, T4 (2.0 μg/kg BW) normalized serum T4, T3, and basal TSH concentrations but failed to normalize TRH-stimulated TSH levels. In patients with slight hypothyroidism and slight enlargement of the sella turcica, T4 (1.84 μg/kg BW) normalized serum T4, T3, and TSH (both basal and TRH stimulated) concentrations. In five patients, an apparent paradoxical increase of basal serum TSH level was found shortly after starting thyroid hormone treatment. It is suggested that pituitary-thyroid interplay during the first 3-6 months of replacement therapy for hypothyroidism varies greatly depending on the severity of hypothyroidism.