Effect of breast-feeding on infant thyroid activity: 3 year follow up--longitudinal study.

The effect of breast-feeding on serum thyroid hormones and TSH was studied in a longitudinal study from birth up to 3 years. T4 was found to be significantly higher in breast-fed than in weaned infants at the age of 4 and 6 months and so was rT3 at the age 4 months. The body weight of breast-fed girls at the age 4 and 6 months was lower than that of formula-fed ones. Breast-feeding at the age 9 months resulted in a decreased serum TSH. Serum T3 of infants weaned before the end of the first week of life was higher than in other groups of infants at the age of 10 weeks. Serum T4 of children 1-2-year-old correlated positively with the age at weaning, provided that they had been breast-fed at least for 60 days. Serum rT3 of 2-year-old children also correlated with the age of weaning. Serum TSH at the age of 3 years was higher in children who had been weaned during first 60 days of life than in those weaned later. It is concluded that breast-feeding possess some immediate effects on thyroid function parameters. Some effects were still detected at the age of 1-3 years.