SAT-449 Factors Associated with Reduced Thyroid Hormones in Cushing Syndrome Patients Before and After Surgical Cure

Background: Hypercortisolemia adversely affects thyroid hormone secretion. We previously described the temporal pattern of thyroid function recovery in 23 patients (1). However, the factors leading to suppression and recovery of the hypothalamic-pituitary-thyroid (HPT) axis in Cushing’s syndrome (CS) are not fully understood. We performed two separate studies to investigate these factors. Methods: In study 1, we examined patients (pts, n=62) with CS who underwent curative surgery and recorded their serum morning and evening cortisol, ACTH, tumor volume and duration of symptoms and 24-hour urine free cortisol (UFC) at baseline and the morning serum free T4, TSH and T3 at six-month intervals after cure. Data were log-transformed and Pearson correlations were performed. Linear mixed models were used to study factors that predict recovery of thyroid function. In study 2, we examined the diurnal variation of TSH by performing hourly TSH measurement between 3—7 PM and 12—4 AM on a cohort of pts (n=45) before surgery. Wilcoxon Signed-Rank method was used for comparisons of mean TSH across time and Pearson correlations were performed on log-transformed data. P values <.05 were considered significant. Results: Study 1: In this larger cohort, we confirmed previous findings of suppressed or low normal fT4 and TSH values with active hypercortisolism, with normalization after cure that reflected changes in the T3:TSH, fT4:TSH and T3:fT4 ratios. There were inverse linear correlations between log10 UFC, serum AM and PM cortisol; and log10 TT3, fT4 and TSH before surgery. Independent negative prognosticators of circulating fT4 recovery included UFC greater than 1000mcg/day (nl: 3.5—45mcg/day), duration of symptoms of less than one year, and ACTH levels greater than 60pg/mL(nl: 5—45pg/mL) Study 2: The nocturnal (12 - 4AM) TSH surge was reduced, so that the difference in day and night TSH values was not statistically significant; this contrasts with the 30—50% nocturnal TSH increase above daytime values seen in healthy subjects. There was an inverse relationship between UFC and nocturnal TSH, daytime TSH and TBG values, but there was no direct relationship between UFC and percent changes in nocturnal TSH values. Conclusions: Our findings suggest that a deficit in TSH stimulation of the thyroid gland may explain the reduction in T3 and T4 levels. There is a dose-response relationship between various measures of hypercortisolemia and both thyroid hormones and the pattern of TSH secretion. Finally, the severity of hypercortisolism correlates with a longer time to recovery of the HPT axis in pts with CS after curative surgery. 1. Shekhar S et al. HPG and HPT Axes in Cushing Syndrome. J Endocr Soc, 3 S1, April May 2019