Patients with Cushing's syndrome (CS) frequently develop hyperthyroidism after surgery due to SITSH (syndrome of inappropriate secretion of TSH) and this SITSH contributed to the symptoms of steroid withdrawal syndrome (SWS). However, the duration of fluctuations in thyroid hormones after surgery for CS remains unknown. The aim of this prospective study was to investigate the clinical course of fluctuation in thyroid hormone level in CS patients after surgery. Thyroid hormone levels [free T3 (FT3), free T4 (FT4) and TSH] and serum cortisol levels were measured before and 1, 3, 6 and 12 months after surgery in 8 patients with active CS (3 pituitary CS and 5 adrenal CS). FT3 levels were above the normal range in 75% of patients up to 6 months after surgery, but returned to the normal range by 12 months. However, TSH levels were not suppressed below the normal range throughout the first 12 months after surgery. Serious symptoms of SWS appeared during the 6-month period after surgery, but disappeared with normalization of thyroid function at 12 months, which was not related to the recovery of function hypothalamus-pituitary-adrenal axis after CS surgery. Therefore, T3 toxicosis could result in deterioration of SWS after surgery for CS. These results indicate that physicians need to take T3 toxicosis into consideration in the pathological evaluation of SWS within 12 months after surgery for CS.