Bone age at birth; method and effect of hypothyroidism.

Bone maturity at birth (or during the neonatal period) can be estimated from the ossified distal femoral epiphyses (FE). In congenital hypothyroidism (CHT) bone maturation is retarded and neonatal bone age reflects the severity of prenatal thyroid failure. In our reference group of 111 healthy infants, the size of these epiphyses depended not only on the age but also on body size. Thus, if bone age is estimated from the size of an epiphysis, the size of the infant is a potential confounder. This problem was avoided by estimating the maturation lag from the difference between the observed and predicted heights of FE (FEHs). Models for predicting FEH were constructed from data from the reference group by multiple linear regression and confirmed in a separate group of 37 healthy infants. In 52 hypothyroid newborns both FEH and FEH lag correlated with serum thyroxine concentration, indicating that FEH can be used as a measure of bone maturation in a population that is fairly homogeneous for (postmenstrual) age and size. Otherwise FEH lag is a better indicator.