Overweight increases risk of first trimester hypothyroxinaemia in iodine‐deficient pregnant women

Hypothyroxinaemia early in pregnancy may impair fetal brain development. Increased body weight has been associated with low thyroxine concentrations in non‐pregnant women. In pregnant women, morbid maternal obesity is a risk factor for thyroid dysfunction. But whether lesser degrees of overweight that are much more common could be a risk factor for hypothyroxinaemia in pregnancy is unclear. The objective of this study was to investigate if overweight increases risk for thyroid dysfunction, and specifically hypothyroxinaemia, in iodine‐deficient pregnant women. We performed a cross‐sectional study at first hospital visit among healthy T hai pregnant women. We measured weight and height, urinary iodine concentration ( UIC ), serum thyroid hormones and thyroglobulin. Pre‐pregnancy weight and relevant dietary factors were determined by questionnaire, and body mass index ( BMI ) was used to classify weight status. Among 514 women (mean gestational age, 11 weeks) with a median UIC of 111 μg dL –1 , indicating mild iodine deficiency, 12% had low free thyroxine ( fT4 ) concentrations: 3% had overt hypothyroidism; 7% had subclinical hypothyroidism; and 8% had isolated hypothyroxinaemia. Based on pre‐pregnancy BMI , 26% of women were overweight or obese. In a multiple regression model, BMI was a negative predictor of fT4 ( β  = −0.20, P  < 0.001). Compared to normal weight women, the prevalence ratio (95% CI ) of a low fT4 in overweight women was 3.64 (2.08–6.37) ( P  < 0.01). Iodine‐deficient pregnant Thai women who are overweight have a 3.6‐fold higher risk of hypothyroxinaemia in the first trimester compared to normal weight women. Targeted screening should consider overweight a potential risk factor for thyroid dysfunction in pregnant women in iodine‐deficient areas.