It has been described alterations of the thyroid function in patients with chronic renal disease without primary disease of the thyroid gland. Nevertheless primary hypothyroidism is more frequent in this population than in other non renal chronic disease. Authors report two cases of hypothyroidism diagnosed in infants with terminal renal failure in the neonatal period, secondary to renal hypoplasia / dysplasia, both in peritoneal dialysis programme. The diagnosis was made by elevated value of thyrotropin (TSH) on the metabolic screening test (TSH/PKU screening test) and confirmed later by measure of serum TSH, total triiodothyronine (T3), total thyroxine (T4), free triiodothyronine (FT3), free thyroxine (FT4) and thyroglobulin (TGB). It was also investigated antithyroid autoantibodies. Both infants initiated hormonal substitution with thyroxin, which have been suspended in one of them by two years of age when thyroid function was normal.