Divergent Patterns of Thyrotropin and Other Thyroidal Parameters in Relationship with the Sex of Healthy Neonates and Infants Less than Two Years Old: A Longitudinal Study

Background: A longitudinal study was conducted in full-term healthy infants who were born between 2015 and 2017 in Athens, Greece, to elucidate the evolution of thyrotropin (TSH) and other thyroidal parameters according to sex, from their day of birth until two years old. Other thyroidal parameters that were taken into account include antithyroid peroxidase antibody (TPO-Ab) and antithyroglobulin antibody (TG-Ab), total triiodothyronine (T3), and free triiodothyronine (fT3), along with total thyroxine (T4) and free thyroxine (fT4). Methods: Blood samples were taken at 5-day intervals from the day of birth until the 31st day of life, and then every 5th month until 2 years of age. All thyroid parameters were measured by electrochemiluminescence immunoassays. The study took place at the Iaso General, Maternity and Gynecological Clinic in Athens, Greece. Results: The sample consisted of 2916 full-term healthy neonates/infants: 1507 (51.7%) boys and 1409 (48.3%) girls. There were no significant differences in TSH levels between boys and girls in all time periods from birth up to 2 years except between 11 and 15 months of age (p = 0.038). Mean TSH levels for boys exhibited much more fluctuation and variability than for girls. In boys we found a significant association between TSH levels and fT4 (p < 0.001), while we found a significant association between TSH levels and T3 in girls (p = 0.045). Furthermore, we found that mean TPO-Ab and TG-Ab levels for boys exhibited larger variability than those for girls. Conclusions: In this study, we were able to plot the development of TSH and other thyroidal parameters by sex from birth up to two years of age. In terms of clinical practice, our findings suggest the need for a re-evaluation of the reference ranges of the studied parameters according to sex, especially in the first months of life and until the first year. Furthermore, our results suggest new optimal ranges for thyroid hormone replacement for that specific period.