Age- and sex-associated differences in the relationship of serum hexosaminidase and T4 levels in human neonates

N-Acetyl-β-glucosaminidase (HEX) activity (total, A and B) and serum thyroxine (T₄) were determined in 22 cord serum samples, 240 serum samples from newborns up to 7 days of life, and 18 samples from infants one month of age, approximately equally divided by sex. Normal values for HEX activity in the neonatal period are given for both sexes. Cord serum values showed no significant sex differences in T₄ levels, total HEX activity, and ratio of A:B isozymes. During the first week of life, highest levels of HEX activity (males: 1378 ± 337 U/ml; females: 1149 ± 178 U/ml; P < 0.01) occur between the first and second day, and coincide with peak serum T₄ levels (males: 15.5 ± 2.9 μg/dl; females: 18.3 ± 3.3 μg/dl; P < 0.025). Although serum HEX activities in males are significantly elevated above female values throughout the first 7 days of life, no sex difference is seen in the (A:B) isozyme ratio (males = 51%:49%; females = 52%:48%). Sera from male infants at one month of age have a mean HEX level of 1734 ± 306 U/ml, which is significantly greater than that at one week of life (1231 ± 190 U/ml) (P < 0.0005). The ratio of A:B, however, remains constant (51%:49% at 1 week; 53%:47% at 1 month). Females show less-pronounced changes in total HEX values between one week (966 ± 155 U/ml) and one month (1174 ± 163 U/ml) (P < 0.005), but exhibit a marked alteration in the (A:B) isozyme ratio (52%:48% at 1 week to 61%:39% at 1 month, P < 0.0005). The percentage of HEX A in females at one month thus approaches prepubertal and adult values. The observed sex differences in HEX activity during the first month of life suggest that not only T₄ but other sex-and/or developmentally related factors as well influence total HEX activity by specific regulation of its isozymes.