Hypothyroidism occurs when the thyroid gland produces insufficient thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), which are essential for regulating metabolism, growth, and development. During pregnancy, these hormones are especially important for maternal metabolism and fetal brain development. Hypothyroidism is the second most common endocrine disorder in pregnancy, affecting approximately 0.3% to 0.5% of pregnant women as overt disease, with subclinical hypothyroidism (elevated TSH, normal free T4) in another 2% to 3%. The primary causes of hypothyroidism include iodine deficiency and autoimmune thyroiditis, with Hashimoto’s thyroiditis being the most common cause in developed countries.
In Hashimoto’s thyroiditis, the immune system attacks the thyroid gland, leading to inflammation and destruction of thyroid cells, reducing hormone production. Symptoms of hypothyroidism, such as fatigue, weight gain, and constipation, often overlap with normal pregnancy changes, making diagnosis challenging. Early detection is critical, as untreated hypothyroidism can result in serious maternal and fetal complications.
During pregnancy, the body’s demand for thyroid hormones increases due to higher metabolic needs and the fetus's reliance on maternal thyroid hormones, particularly in the first trimester when the fetal thyroid is not fully developed. Maternal thyroid hormones are essential for proper fetal brain and nervous system development, making it crucial to maintain adequate thyroid hormone levels for the health of both the mother and baby. Levothyroxine requirements rise early: women already on replacement should increase their dose by about 25% to 30% as soon as pregnancy is confirmed (a practical method is two extra tablets per week, a 29% increase), with TSH checked every 4 weeks through mid-gestation and the dose titrated to the lower half of the trimester-specific reference range. Levothyroxine is separated from iron- and calcium-containing prenatal vitamins by about 4 hours, and the dose returns to the pre-pregnancy amount after delivery. Overt hypothyroidism is always treated; the ATA also recommends levothyroxine for subclinical hypothyroidism when thyroid peroxidase antibodies are present and TSH is above the reference range.
