The Relationship Between Levothyroxine and Gestational Weight Gain in Subclinical Hypothyroidism Compared to Euthyroid Individuals

Abstract

Background: Untreated subclinical hypothyroidism (SCH) can lead to complications during pregnancy. One significant concern associated with pregnancy is excessive weight gain. Given the link between hypothyroidism and obesity, we aimed to investigate the relationship between the dose of levothyroxine and gestational weight gain (GWG), for which we did not find any reports. Additionally, we measured the differences in GWG between women with and without SCH who were monitored until the end of their pregnancies. Methods: Primiparous women (N = 245) were selected using cluster sampling and entered into a prospective cohort study. Subclinical hypothyroidism (SCH) was determined so that participants could be divided into two groups, with and without SCH, and followed until the end of pregnancy. Levothyroxine dose was used as an independent variable, and maternal weights at 6–10, 18, 26, 33, 36 weeks, and at term (37–41) were determined as dependent (outcome) variables. Thyroid-stimulating hormone (TSH) levels during the first trimester and pre-pregnancy body mass index (BMI) were also recorded as key influencing variables. Data were collected via interviews and prenatal records from community health centers. Ten specialists confirmed the face validity of the questionnaire. To enhance reliability, all interviewers received training under the same conditions. Additionally, since the information was recorded using fixed and standardized forms, the reliability of the data had already been established. Results: After adjusting for time (weight measurement in different weeks of pregnancy) and pre-pregnancy BMI, the average dose of levothyroxine was found to be correlated with gestational weight gain (GWG) (β = 0.036, SE = 0.016, P = 0.025). Additionally, there were no significant differences in total GWG between individuals with subclinical hypothyroidism (SCH) and euthyroid concerning their levothyroxine use (yes or no) (β = 0.812, SE = 0.997, P = 0.415). Conclusion: We found a positive correlation between levothyroxine dosage and weight gain during pregnancy, which is a valuable result. Additionally, total weight gain in participants with subclinical hypothyroidism treated with levothyroxine did not differ statistically from that of euthyroid subjects. © 2026 The Author(s); Published by Kerman University of Medical Sciences.