Subclinical thiamine deficiency in third-trimester pregnancy despite routine antenatal supplementation: a cross-sectional study in an urban setting, Bangkok, Thailand

Thiamine deficiency disorders (TDD) remain a preventable yet underrecognized contributor to maternal and infant morbidity, particularly in low- and middle-income countries. During pregnancy, thiamine deficiency may go undetected despite routine antenatal care and can lead to serious neurological and cardiac complications in breastfed infants. However, data on thiamine status in urban populations receiving standard antenatal supplementation are limited. This study aimed to determine the prevalence of thiamine deficiency in third-trimester pregnant women and to identify associated factors and clinical manifestations in an urban setting of Thailand. This cross-sectional study enrolled pregnant women in the third-trimester (≥ 28 weeks of gestation) attending antenatal care in Bangkok, Thailand. Demographic characteristics, obstetric history, dietary habits, medication and supplement use, and TDD-related symptoms were collected using a structured questionnaire. Functional thiamine status was assessed using the erythrocyte transketolase activity coefficient (ETKAC), with abnormal thiamine status defined as ETKAC > 1.25. Penalized logistic regression with Firth’s correction was used to identify factors associated with abnormal ETKAC. A total of 328 participants (mean age 28.9 years; mean gestational age 31.1 weeks) were included. Although 97.9% reported receiving at least one antenatal supplement, only 53.1% received formulations containing thiamine. Based on ETKAC, 17.7% demonstrated biochemical thiamine deficiency (ETKAC > 1.25). Consumption of refined carbohydrates without thiamine containing supplement was independently associated with deficiency (aOR 4.20, 95% CI 1.30–17.87). Clinical symptoms were nonspecific and largely similar between groups, except for reduced concentration or memory problems, which was more common among participants with thiamine deficiency ( p = 0.007). These findings suggest that despite high antenatal supplementation coverage, nearly one-fifth of third-trimester pregnant women demonstrated biochemical evidence of subclinical thiamine deficiency. Routine clinical assessment was insufficient to identify affected individuals. These findings suggest that current antenatal supplementation strategies may be inadequate to ensure optimal thiamine status during late pregnancy and highlight the need to re-evaluate micronutrient policies in similar urban settings.