Patterns off recovery of the hypothalamic-pituitary-thyroid axis in patients taken of chronic thyroid therapy.

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To determine the patterns of recovery of the hypothalamic-pituitary-thyroid axis following long-term thyroid hormone therapy, TRH tests were performed on 8 euthyroid nongoitrous patients, 5 euthyroid goitrous patients, and 5 hypothyroid patients while they were taking full doses of thyroid hormone and 3, 7, 10, 14, 17, 21, 28, 35, 42, 49, and 56 days after stopping it. Serum TSH, T3, and T4 were measured before and at multiple intervals over a 4-h period after giving 500 mug TRH iv. In euthyroid non-goitrous patients, the mean duration of suppressed TSH response to TRH (maximum deltaTSH less than 8 muU/ml) was 12 +/- 4 (SE) days after stopping thyroid hormone and the mean time to recovery of normal TSH response to TRH (maximum deltaTSH greater than 8 muU/ml) was 16 +/- 5 days. None of the euthyroid nongoitrous patients ever hyperresponded to TRH; their average maximal deltaTSH was 24.5 +/- 2.2 muU/ml. Serum T4 fell below normal in 4 euthyroid non-goitrous patients, reaching lowest values at 4 to 28 days. While serum T4 was low, deltaTSH was subnormal. Normal increments of T4 and T3 after TRH occurred at 19 +/- 5 and 22 +/- 6 days, respectively. In the 5 goitrous patients, patterns of recovery of pituitary and thyroid function assessed by the same parameters were much less consistent. In the 5 hypothyroid patients, the mean duration of suppressed basal TSH and suppressed deltaTSH was 13 +/- 3 days; mean time to attain a supranormal basal TSH (greater than 8 muU/ml) was 16 +/- 4 days and to reach a supranormal deltaTSH (greater than 38 muU/ml) after TRH was 29 +/- 8 days. Following prolonged thyroid therapy in euthyroid patients, recovery of normal TSH responsiveness to TRH preceded recovery of the normal T3 and T4 response to TRH by 3 to 6 days. Basal serum TSH may be used to differentiate euthyroid from hypothyroid patients 35 days after withdrawal of thyroid therapy; the response to TRH does not improve this differentiation.