Thyroid dysfunction highly prevalent in Cushing’s syndrome
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Central hypothyroidism is prevalent in about 1 in 2 adults with Cushing’s syndrome, and thyroid function can be restored after curative surgery for most patients, according to study findings.

We do not fully understand how hypercortisolism causes central hypothyroidism or what factors influence recovery of the hypothalamic-pituitary-thyroid axis. Researchers evaluated thyroid function during and after cure of Cushing syndrome (CS).

They performed a retrospective cohort study of adult patients with CS seen from 2005 to 2018 (cohort 1, c1, n = 68) or 1985 to 1994 (cohort 2, c2, n = 55) at a clinical research center. Urine (UFC) and diurnal serum cortisol (F: ~8 AM and ~midnight [PM]), morning 3,5,3-triiodothyronine (T3), free thyroxine (FT4), and thyrotropin (TSH) (c1) or hourly TSH from 1500 to 1900 h (day) and 2400 to 04000 h (night) (c2), were measured before and after curative surgery.

-- While hypercortisolemic, 53% of c1 had central hypothyroidism (low/low normal FT4+unelevated TSH).

-- Of those followed long term, 31% and 44% had initially subnormal FT4 and T3, respectively, which normalized 6 to 12 months after cure.

-- Hypogonadism was more frequent in hypothyroid (69%) compared to euthyroid (13%) patients.

-- Duration of symptoms, morning and midnight F, adrenocorticotropin, and UFC were inversely related to TSH, FT4, and/or T3 levels.

-- In c2, the nocturnal surge of TSH (mIU/L) was subnormal before and normal at a mean of 8 months after cure.

-- UFC greater than or equal to 1000 microg/day was an independent adverse prognostic marker of time to thyroid hormone recovery.

Conclusively, abnormal thyroid function, likely mediated by subnormal nocturnal TSH, is prevalent in Cushing syndrome and is reversible after cure.