Breast feeding and formula feeding affect differently plasma thyroid hormone concentrations in infant baboons.

We tested the hypothesis that plasma thyroid hormone levels in infant baboons are differentially affected by breast-feeding and nursery rearing. Infant baboons were breast-fed (n = 17), or fed formulas with a high polyunsaturated to saturated fat ratio (P/S) (n = 13), or a low P/S formula (n = 14). Plasma total triiodothyronine (T3) and free T3 (fT3) concentrations increased from 4 to 14 weeks of age in infants fed either formula, but decreased in breast-fed infants. At 9 and 14 weeks of age, infants fed the formulas averaged 31% higher T3 and 53% higher fT3 concentrations compared with breast-fed infants. Although T3 concentrations of the high P/S formula group compared with the low P/S formula group were not different at any single age, from 4 to 14 weeks the T3 averaged 19% higher in the high P/S formula group. Plasma total thyroxine (T4) or free T4 (fT4) concentrations were not significantly different among the diet groups during the preweaning period. In summary, formula-fed infants maintained higher plasma T3 and fT3 levels compared with breast-fed infants in the late preweaning period.