ACTH-secreting pituitary microadenomas are associated with a higher prevalence of central hypothyroidism compared to...

Unlike pituitary macroadenomas, microadenomas (micros) are not commonly associated with hypopituitarism. In clinical practice, we have observed that patients with ACTH-secreting micros have a higher than expected prevalence of central hypothyroidism (HT), and we speculated that this effect might be because of glucocorticoid-induced suppression of the hypothalamic-pituitary-thyroid axis.

To determine whether there is a difference in the prevalence of central HT among ACTH micros compared to other types of microadenoma, and if so, to investigate whether this is directly related to the degree of hypercortisolism.

Retrospective study of 149 newly diagnosed patients with pituitary micros: 34 ACTH-secreting, 72 prolactin-secreting (PRLomas) and 43 clinically nonfunctioning adenomas (NFAs).

Prevalence of central HT, correlation between normalized free T4 or TSH vs normalized urinary free cortisol (UFC) or salivary cortisol.

The prevalence of central HT was significantly higher in the ACTH compared to the non-ACTH adenomas: 18% (ACTH), 1% (PRL) and 0% (NFAs). The mean normalized free T4 was lower in the ACTH micros compared to the non-ACTH micros (1·29 ± 0·06 vs 1·50 ± 0·23, P = 0·0001). There was no correlation between the degree of hypercortisolism, as reflected by 24-h urine free cortisol and salivary cortisol, and free T4 or TSH levels among the ACTH adenomas. Similarly, there were no differences in mean UFC or salivary cortisol between ACTH adenomas with and without central HT. Following transsphenoidal adenomectomy, central HT recovered in three of six patients with ACTH micros.

These findings suggest that patients with ACTH-secreting microadenomas should always, at a minimum, undergo testing for central HT. However, given the potential for recovery of thyroid function following cure of Cushing disease, we recommend withholding thyroid hormone replacement until after pituitary surgery.