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Although anemia is a common finding in adult hypothyroid patients, there are no studies on anemia in hypothyroid infants. The aim of this study, therefore, was to review the hematologic status during the first year of life in 50 infants with congenital hypothyroidism detected through the regional neonatal screening program. The mean age at diagnosis was 23.7 +/- 6.5 days and treatment was initially begun with a mean L-thyroxine dose of 6.8 +/- 1.3 micrograms/kg/day. Clinical and haematological assessments were performed at diagnosis, 3, 6 and 12 months of age. The patients were divided in 2 groups based on whether T4 serum concentration at diagnosis was < 3 micrograms/dl (Group A) or > or = 3 micrograms/dl (Group B). Data for hemoglobin (Hb), hematocrit (Ht), red cells count (RCC), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), serum iron and ferritin were expressed as Standard Deviation Score (SDS). Although at diagnosis the mean value of Hb-SDS, Ht-SDS and RCC-SDS were in the low-normal range in both groups, at 3 months of age the values in Group A (Hb -1.9 +/- 0.79; Ht -2.34 +/- 1.02; RCC -1.56 +/- 1.25) were significantly lower than in Group B (Hb -1.14 +/- 0.78, p < 0.005; Ht -1.59 +/- 0.94, p < 0.05; RCC -0.55 +/- 1.32, p < 0.02). A rise of the Hb, Ht and RCC values was observed in both groups from 6 to 12 months. The mean values of MCV-SDS and MCH-SDS were in the normal range at diagnosis in both groups, decrease progressively at 3 and 6 months and returned to normal at 12 months of age; no differences were found between the 2 groups at any time. Mean Hb levels at 3 months of age were correlated with mean serum T4 at diagnosis (r = 0.30, p < 0.05). The present results indicate that anemia is a frequent finding in infants with congenital hypothyroidism and is depended on the degree of neonatal hypothyroidism and imply that hypothyroidism during development may produce persisting changes even after thyroid replacement has begun.