Associations between urinary phthalate monoesters and thyroid hormones in pregnant women

Abstract

Maternal hypothyroidism during pregnancy can cause adverse effects in the fetus. Scientific evidence has shown that probable thyroid-like function of some phthalates in vitro and in vivo, and phthalates exposure, can begin in utero. This study investigated the association between phthalate exposure and thyroid hormones in pregnant women. Serum and spot urine samples were collected from 76 Taiwanese pregnant women at second trimester. Thyroid hormones, including thyroid-stimulating hormone (TSH), triiodothyronine (T₃), thyroxine (T₄) and free T₄ (FT₄) were analysed in serum samples, and five urinary phthalate monoesters, including mono butyl phthalate (MBP), monoethyl phthalate (MEP) and mono ethylhexyl phthalate (MEHP), were measured. Urinary MBP, MEP and MEHP, the median levels of which were 81.8, 27.7 and 20.6 ng/ml, respectively, were the predominant substances in the urinary phthalate monoesters. Significant mild negative correlations were found between T₄ and urinary MBP (R = −0.248, P < 0.05), and between FT₄ and urinary MBP (R = −0.368, P < 0.05). After adjusting for age, BMI and gestation, urinary MBP levels showed negative associations with FT₄ and T₄ (FT₄: β = −0.110, P < 0.001; T₄: β=−0.112, P = 0.003). Exposure to di-n-butyl phthalate (DBP) may affect thyroid activity in pregnant women, but how DBP affects thyroid function is unclear. Further studies are needed to elucidate the mechanism of action and to investigate whether any other factors related to DBP exposure alter the thyroid function.