Objective: Variations in thyroid function are known to be associated with changes in adrenocortical activity. Previous studies in animals have suggested that long-standing hyperthyroidism may be associated with diminished adrenal functional reserve despite a continuing hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis. In humans, there has been no direct assessment of adrenal secretory reserve in clinical thyrotoxicosis. This study aimed to assess adrenocortical reserve in response to low-dose ACTH, following dexamethasone suppression, in patients with severe thyrotoxicosis. Design and methods: Ten patients (four men and six women, 30-45 years) with severe long-standing thyrotoxicosis due to Graves' disease (n = 6) or toxic nodular goitre (n = 4) were studied at diagnosis and again when in a stable euthyroid state following drug therapy for 8-12 months. All patients underwent ACTH stimulation tests at 0800 h with ACTH 1-24 (Cortrosyn; 0.1 mg/kg body weight, i.v.) following overnight suppression of the HPA axis with dexamethasone (1 mg per os at 2300 h). Serum cortisol was assayed at ¹15, 0, 15, 30, 60 and 90 min after the administration of ACTH. Results: The mean (Ϯ S.D.) peak and delta cortisol responses to ACTH (634.5 Ϯ 164 nmol/l and 618 Ϯ 196 nmol/l respectively), as well as the net area under the response curve (36 769 Ϯ 12 188 nmol/l × min) in the hyperthyroid patients were significantly lower compared with the values when the same patients were euthyroid (911 Ϯ 157 nmol/l, 905 Ϯ 160 nmol/l and 57 652 Ϯ 10 128 nmol/l × min respectively; P < 0.005). Subnormal peak cortisol responses (<500 nmol/l) were observed in two severely toxic patients. The findings were independent of the cause of thyrotoxicosis. Conclusion: In patients with severe thyrotoxicosis, cortisol secretion in response to low-dose ACTH stimulation, following dexamethasone suppression, is lower in the hyperthyroid than in the euthyroid state. It appears that thyrotoxicosis is associated with subtle impairment of adrenocortical reserve.
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