Thyroid hormone status in umbilical cord serum is associated with male anogenital distance

Context: Animal experiments have shown that thyroid hormones played a crucial role in embryonic gonad development. In human adults and adolescents, thyroid function affects sex hormones and male reproductive functions. ... [ view full abstract ]

Context: Animal experiments have shown that thyroid hormones played a crucial role in embryonic gonad development. In human adults and adolescents, thyroid function affects sex hormones and male reproductive functions.
Objective: The aim was to study the association of thyroid hormones (THs) with sexually dimorphic genital development and fetal growth in human infants.
Participants: Six hundred twenty pregnant women were recruited from two local hospitals. THs and AGDs were available for 616 singleton newborns.
Main Outcome Measures: Thyrotropin (TSH), free 3,5,3′-triiodothyronine (FT3) and free thyroxine (FT4) levels in cord blood serum, anogenital distance (AGD), birth weight, birth length, birth body mass index (BMI) and head circumference in male and female neonates.
Results: Longer AGD in male newborns was observed with higher cord serum FT3 [β 1.36 mm (95% CI 0.58–2.13) for 1 pmol/L FT3], FT4 [β 0.12 mm (95% CI 0.00–0.25) for 1 pmol/L FT4], TSH [β 3.14 mm (95% CI 0.65–5.63)for a 10-fold TSH increase], and lower FT4/FT3 ratio [β -0.11 mm (95% CI -0.20– -0.02) for doubling FT4/FT3 ratio]. The relation between TSH and birth weight or birth length were different by second hand smoke exposure. Second hand smoke exposure had an effect modification, with interaction P value 0.039 and 0.010, respectively.
Conclusions: In the absence of overt thyroid dysfunction, THs are associated with AGD in male newborns. THs may affect fetal genital development in a sexually dimorphic pattern, whereas the effects on body size and head circumference may be modified by maternal second hand smoke exposure.