Risk factors for discontinuation of thyroid hormone replacement therapy in early pregnancy: a study from the Norwegian Mother and Child Cohort Study and the Medical Birth Registry of Norway

Introduction Approximately 3–5% of pregnant women have hypothyroidism. Despite the potential impact of untreated hypothyroidism on infant neurodevelopment, few studies have investigated the risk factors associated with discontinuation of thyroid hormone replacement therapy (THRT) in pregnancy. We aimed to identify such factors in a population of women using THRT prior to pregnancy. Material and methods Data from the Norwegian Mother and Child Cohort Study were linked to records in the Medical Birth Registry of Norway. Pregnant women with hypothyroidism prior to pregnancy were categorized as discontinuers or continuers of THRT in pregnancy. The main analysis used generalized estimating equations based on multiply imputed data. Results Of 86 848 enrolled pregnant women, 2720 (3.2%) had a medically confirmed thyroid disorder and/or reported use of thyroid therapy. More than half (n = 1587; 57.8%) used THRT prior to pregnancy; of these, 207 (13.0%) discontinued and 1380 (86.9%) continued THRT during early pregnancy. Having a non‐medicated mental disorder [odds ratio (OR) 1.64, 95% CI 1.03–2.63] and non‐compliance with recommended nutritional supplementation (OR 2.51, 95% CI 1.82–3.47) increased the odds of discontinuing THRT. Women medicated for somatic comorbidities (OR 0.56, 95% CI 0.33–0.98) had a 44% decreased odds of discontinuing THRT. Conclusions In Norway, around 13% of women with hypothyroidism discontinue THRT in early pregnancy. For discontinuers, non‐medicated mental comorbidity and non‐compliance with nutritional supplements presented increased risk, whereas having a medicated somatic disorder was protective. Health professionals advising women with hypothyroidism should be aware of risk factors associated with THRT discontinuation.