Improvement of hypothyroidism after glucocorticoid replacement in isolated adrenocorticotropin deficiency

We report a 50-year-old female who suffered from reversible hypothyroidism accompanied by isolated ACTH deficiency. There were no findings indicating a complication of autoimmune thyroiditis. Replacement of maintenance dose of glucocorticoid not only led to improvement of thyroid function, but also caused a transient decrease in T3 and an increase in reverse T3, suggesting that chronic cortisol deficiency may impair thyroid function, and that the maintenance dose, as well as pharmacological doses of glucocorticoids may influence T4 deiodination. The findings of this case suggest that thyroid function should be re-evaluated to avoid unnecessary replacement of thyroid hormone, a few months after glucocorticoid replacement.