The degree of thyroid impairment and the effects on growth have not been investigated in children with Cushing's disease. We followed the thyroid function of 24 children and adolescents (12 males and 12 females) with CD (age, 12.9 ± 3.2 years; mean ± SD), who were successfully treated by transsphenoidal surgery. Patients were evaluated before, and 3, 6, and 12 months after TSS. Analysis of variance and linear correlation were performed between thyroid function tests and body weight and mass index and bone age. Preoperative free thyroxine levels (1.37 ± 0.03 ng/dl) were significantly higher than those at 3 months (1.17 ± 0.05 ng/dl, p < 0.05), but similar to those at 6 and 12 months postoperatively. Preoperative T3 (114.2 ± 7.7 ng/dl) and TSH (1.36 ± 0.2 IU/ml) were significantly lower than the postoperative values at 3 (158.9 ± 6.8 and 2.3 ± 0.3, respectively), 6 (159.1 ± 10.8 and 2.5 ± 0.3, respectively), and 12 months (136 ± 6.5 and 2.2 ± 0.3, respectively) (all p < 0.05). One patient had frank hypothyroidism (fT4 < 1 ng/dl) before surgery. Five additional patients had secondary hypothyroidism in the immediate postsurgical period; two of them had normal thyroid function 2 and 3 years postoperatively. One patient has remained hypothyroid for more than 5 years since surgery. No significant correlation was found between thyroid function and body weight, BMI, or BA. We conclude that hypothyroidism was an infrequent complication of CD and TSS. Mild suppression of thyroid function occurs in most children and adolescents with CD before and in the first few months after TSS, but it fully resolves after 6 months and does not correlate with the growth delay and obesity of these patients. (J Pediatr 1997;131:905-9)