Infant feeding, fetal growth and adult thyroid function.

Prolonged breast-feeding in humans is associated with increased low-density lipoprotein cholesterol and higher death rates from ischaemic heart disease in adult life. The reasons for this link are unclear. A possible explanation is that thyroid hormones present in breast milk and absorbed by the suckling infant could, by the process of hormonal imprinting, permanently down-regulate the set point of thyroid homeostasis. Thyroid hormones influence cholesterol metabolism, and could explain the link between infant feeding and the regulation of cholesterol levels in the adult. We therefore investigated whether infant feeding was related to adult thyroid function in 303 women aged 60-71 years who were born in the county of Hertfordshire, UK, where birthweight, the weight at 1 year and the method of infant feeding had been recorded routinely. Free thyroxine (FT4) concentrations but not free triiodothyronine (FT3) or thyrotrophin (TSH) were increased in the women who, as infants, had been breast-fed beyond 1 year of age (p < 0.01). In women who were bottle-fed, with or without breast-feeding, serum TSH rose and FT4 fell with increasing birthweight (p = 0.01 and p = 0.04, respectively). Although the metabolic significance of these findings is unclear, they suggest that the set point of thyroid function in the adult is determined by fetal growth and infant feeding.