This study deals with the relationship between the occurrence of hypercalcemia and the administration of prophylactic doses of vitamin D in children with hypothyroidism, before and during L-thyroxine (LT4) treatment. The goal of the study was to determine the dosage of vitamin D necessary to prevent rickets without inducing hypercalcemia. There was a 23% prevalence of hypercalcemia at the time of the diagnosis of hypothyroidism by screening whereas it was 21% in the children who were not given vitamin D during the first 3 months of LT4 treatment. This figure was significantly higher in those who were given vitamin D during the first 3 months of treatment and reached 70%. However, one of the 19 children not given vitamin D presented with biological signs evoking vitamin D deficiency. In conclusion, in hypothyroid infants, vitamin D should be administered carefully during the first 6 months of treatment and restricted to children at risk for developing vitamin D deficiency.