Longitudinal study of hormonal and physical development in young twins.
Information on the correlation of normative reproductive hormone levels with physical development (Tanner stages) during puberty and on the influences of genes and environment on variation in these hormones and Tanner stages is limited. One hundred twelve healthy 9-year-old twin pairs (n = 224) took part in a longitudinal study, of which 89 pairs participated again at age 12 years (n = 178). Morning urinary LH, FSH, estradiol, and salivary testosterone levels, determined by competitive immunoassays, were measured. Tanner stages were determined through physical examination. Over the 3-year interval, all hormone levels showed a 2- to 9-fold increase. LH and FSH at age 9 years predicted sex-specific Tanner stages at age 12 years in both boys and girls. Most of the associations between hormone levels at age 9 years and physical development at 12 years were explained by genetic influences. FSH in 9-year-old boys correlated with all hormone levels and Tanner stages at age 12 years. Moderate to high heritability estimates were found for hormone levels at both ages and in both sexes. In girls a shift from environmental (age 9 years) to genetic influences (age 12 years) was found for estradiol and pubic hair development, and for breast development a shift in the opposite direction was seen. During development LH and FSH (and testosterone in boys) levels predict secondary sexual characteristics in boys and girls 3 years later. These correlations are largely due to genes that are involved in both early pubertal hormone levels and subsequent physical development.