7385 Prevalence of Central Hypothyroidism in Cushing Syndrome and Thyroid Function Tests After Cushing Treatment

Abstract Disclosure: T. Kunavisarut: None. A. Sriwijitkamol: None. Cushing syndrome can be associated with thyroid dysfunction. We investigated the association between Cushing syndrome and thyroid dysfunction. We conducted a retrospective study at Siriraj Hospital in Thailand and analyzed thyroid function tests in 80 Cushing's patients. The inclusion criteria were patients diagnosed with Cushing syndrome with thyroid function tests recorded at diagnosis of Cushing syndrome. Patients who had an abnormal thyroid function test at diagnosis of Cushing syndrome must have a thyroid function test after treatment of Cushing syndrome. Of the 80 patients, sixty-five patients (81.2%) had normal thyroid function at diagnosis, 12 patients (15%) had central hypothyroidism and three patients (3.8%) had subclinical hyperthyroidism.Interestingly, among 12 patients who had central hypothyroidism, eight patients (66.7%) were diagnosed with Cushing disease. Three patients (25%) were diagnosed with adrenal Cushing syndrome and one patient (8.3%) was diagnosed with ectopic Cushing syndrome. This finding suggested that central hypothyroidism may not be exclusively due to the mass effect of the pituitary tumor on the normal pituitary because adrenal Cushing syndrome and ectopic Cushing syndrome can also cause central hypothyroidism. Among 12 patients with central hypothyroidism at diagnosis of Cushing syndrome, nine patients (75%) normalized their thyroid function after remission of Cushing syndrome without Levothyroxine replacement. Moreover, patients with central hypothyroidism had significantly higher median 24-hour urine-free cortisol levels than euthyroid patients (1030.5 ug/day vs. 404.5 ug/day, p=0.017), indicating a potential correlation between cortisol excess and low FT4 levels. These findings suggest that hypercortisolism in Cushing syndrome can suppress the hypothalamic-pituitary-thyroid axis, but this suppression is often reversible with successful Cushing's treatment. This study highlights the importance of central hypothyroidism, which can occur in all types of Cushing syndrome. If the patients did not have hypothyroid symptoms, Levothyroxine replacement may not be necessary. For patients who already received Levothyroxine and had normal thyroid function tests, Levothyroxine can be carefully tapered and potentially discontinued because successful Cushing syndrome treatment can ultimately normalize thyroid function tests. Presentation: 6/1/2024