OBJECTIVE: The abnormalities reported in some thyroid function tests in children with renal disease could be adaptive phenomena, shared by a variety of other nonthyroidal illnesses, or could reflect hypothyroidism. STUDY DESIGN: To answer this question, we studied thyroid function and serum thyroid binding proteins in 36 prepubertal and 23 pubertal patients with renal disease receiving three different therapies: conservative treatment, hemodialysis, and care after renal transplantation. RESULTS: During prepuberty, the serum concentration thyroxine binding globulin (mean ± SE) in the three groups of patients (294 ± 18, 303 ± 18, and 323 ± 16 nmol/L, respectively) was significantly lower than in prepubertal control subjects (451 ± 71 nmol/L). Only in prepubertal patients after renal transplantation (3583 ± 573 nmol/L) were serum thyroxine binding prealbumin values lower than in respective control subjects (5999 ± 908 nmol/L). The serum total thyroxine concentration in the three groups of patients (108 ± 41.9, 121 ± 5.7, and 123 ± 5.5 nmol/L, respectively) was significantly lower than in prepubertal control subjects (149 ± 10 nmol/L), whereas serum free thyroxine and serum albumin-bound thyroxine concentrations were similar to those in control subjects. The serum total triiodothyronine level in the three groups of patients (2.29 ± 0.82, 2.13 ± 0.13, and 2.01 ± 0.20 nmol/L, respectively) was significantly lower than in prepubertal control subjects (3.04 ± 0.24 nmol/L), whereas serum levels of free triiodothyronine and serum albumin-bound triiodothyronine were similar to those in prepubertal control subjects. During puberty, serum thyroxine binding globulin and serum thyroxine binding prealbumin levels in the three groups of patients were not statistically different from those in pubertal control subjects (309 ± 47 and 4950 ± 1230 nmol/L, respectively). Serum levels of total thy roxine, free thyroxine, albumin-bound thyroxine, total triiodothyronine, free triiodothyronine, and albumin-bound triiodothyronine were similar to those in pubertal control subjects except for pubertal patients undergoing hemodialysis. In all clinical groups the basal serum thyrotropin concentration was similar to those in respective control subjects. The frequency of goiter was increased in patients undergoing hemodialysis, probably as a result of iodide washout with dialysis. CONCLUSION: Children and adolescents with chronic renal insufficiency or end-stage renal disease or after renal transplantation do not have a primary abnormality of thyroid function and therefore are not candidates for thyroid hormone treatment. (J PEDIATR 1996;128:784-90)