Preliminary Evidence of Potential Association Between Exposure to Estriol-Containing Skincare Cosmetics Sold in China and Autoimmune Thyroid Disease

<b><i>Background:</i></b> Estrogen may contribute to a higher prevalence of autoimmune thyroid disease (AITD) in women. However, it is unknown whether estrogen-containing cosmetics may be associated with AITD. This study aimed to (1) measure the estrogen concentration in skincare cosmetics sold in China, (2) examine the thyroid autoantibody levels in nude mice after dermal estrogen application, and (3) explore the relationship between skincare cosmetic use and AITD in the Chinese population in a population-based cross-sectional study and hospital-based case-control study. <b><i>Methods:</i></b> Skincare cosmetics of variable type and price were collected from various Chinese retail merchants and online shopping platforms to measure the respective estrogen concentrations in the products. Nude mice were smeared with estrogen on the skin to assess their effects on the thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb) levels. A cross-sectional study was conducted in four districts or counties to compare differences in serum TPOAb, TgAb, and thyroid function between cosmetic users and nonusers. A case-control study was conducted in four district hospitals and two city hospitals to explore the association between cosmetic use and AITD. <b><i>Results:</i></b> Estriol was detected in 73 of 240 skincare cosmetic products, with a median concentration of 34.58 µg/g. In animal experiments, serum thyroid autoantibodies were significantly increased in mice smeared with (low or high) estriol-containing creams compared with controls (TPOAb, 31.07 ± 6.89 ng/L or 32.22 ± 8.92 ng/L vs. 21.27 ± 6.32 ng/L, <i>p</i> < 0.05; TgAb, 20.94 ± 6.12 ng/L or 19.56 ± 8.35 ng/L vs. 11.28 ± 1.43 ng/L, <i>p</i> < 0.05). In a cross-sectional study, no difference was found in TPOAb, TgAb, and thyroid function between skincare cosmetic users and nonusers (<i>p</i> > 0.05). However, the case-control study revealed a significant association between skincare cosmetics use and the presence of AITD (odds ratio = 1.577, confidence intervals 1.037-2.400, <i>p</i> = 0.033) after adjusting for other related variables. <b><i>Conclusions:</i></b> Estriol was found in some skincare cosmetics sold in China. Dermal estriol application could induce thyroid autoimmune reactions in nude mice. Preliminary population-based study data suggest that skincare cosmetic use may be associated with AITD. Prospective cohort studies or randomized controlled trials are needed in the future to confirm a potential association between estriol in skincare cosmetic use and AITD.