Abstract. Luteinizing hormone (LH) bioactivity was determined by an in vitro microbioassay in 65 plasma samples from 26 infants and young children between 10 days and 6.5 years of age. In addition LH was measured by radioimmunoassay. For both, LH preparation LER-907 was used as standard.
Biologically measured LH values (bio-LH) were always higher than radioimmunologically determined LH values (RIA-LH) as reflected by bio/RIA ratios greater then 1.0.
In male infants bio-LH was elevated up to 7 months of age. Thereafter, it decreased and remained low over the age range of our study. In female infants bio-LH was also elevated, but decreased after 1 month and showed a slight tendency to rise in the age group 3–6.5 years.
The results of RIA-LH showed approximately the same pattern, but at a lower level. In boys RIA-LH levels were highest around 1 month of age, decreased steadily between 2 and 7 months and remained constant thereafter. Girls had rather constant RIA-LH values between 2 months and 3 years of age. In contrast to bio-LH, there is a clear-cut drop of RIA-LH in the age group 3–6.5 years, resulting in a statistically significant increase of the bio/RIA ratio.
In boys the time course of the bio-LH changes coincides with the known elevation of testosterone during the first months of life.
In girls the elevated bio-LH levels observed during the first month are not so far associated with a known steroid correlate.
Our study shows an increased biological activity of circulating LH and a marked dissociation of biologically and radioimmunologically active LH during early infancy, analogous to observations during puberty.