Hypothyroidism is a common endocrine condition where the thyroid gland does not produce sufficient amounts of its hormones, primarily thyroxine (T4). This underactive thyroid state affects between two and three out of every 100 people and is significantly more common in women. Thyroid hormones regulate the body’s metabolism and are necessary for numerous functions. Maintaining healthy thyroid function is important for a successful pregnancy outcome.
Establishing the Link Between Hypothyroidism and Pregnancy Loss
Uncontrolled hypothyroidism increases the risk of miscarriage, particularly during the first trimester. Studies show that higher levels of Thyroid-Stimulating Hormone (TSH) are associated with a greater chance of pregnancy loss. This risk is dose-dependent; the more severe the thyroid hormone deficiency, the higher the likelihood of a miscarriage.
Inadequately treated hypothyroidism creates an environment less hospitable for the developing embryo. Screening is recommended for women with risk factors due to the link between poor thyroid control and adverse outcomes. Correcting the hormone imbalance reduces pregnancy loss rates to levels comparable to those in women with normal thyroid function.
The Role of Thyroid Hormones in Fetal and Placental Development
Maternal thyroxine (T4) plays a fundamental part in establishing and maintaining a pregnancy in its earliest stages. Before the fetus can produce its own thyroid hormone, which begins around 16 to 20 weeks of gestation, it is completely dependent on the mother’s supply of T4. This maternal T4 is actively transported across the placenta to the developing embryo.
This early supply of maternal T4 is necessary for the proper development of the fetal central nervous system and for the formation and function of the placenta itself. The placenta requires adequate thyroid hormone for its integrity and implantation. A deficiency in maternal T4 can disrupt these processes, leading to placental dysfunction and subsequent failure of the pregnancy. The placental tissue contains enzymes that regulate the amount of T4 reaching the fetus, but this system cannot fully compensate when the maternal supply is severely inadequate.
Identifying Risk: Overt vs. Subclinical Hypothyroidism and Autoimmunity
Hypothyroidism in pregnancy is classified based on blood test results, which helps determine the level of risk and the need for treatment. Overt hypothyroidism, the more severe form, is defined by an elevated TSH level accompanied by a low level of Free T4 (the active thyroid hormone). This condition presents the highest risk for adverse pregnancy outcomes, including miscarriage.
A milder form is subclinical hypothyroidism, characterized by an elevated TSH level above the trimester-specific reference range, but with a Free T4 level that remains within the normal range. While the risk posed by subclinical hypothyroidism is lower than that of the overt form, it still represents a significant threat to a successful pregnancy outcome. Up to 20% of women with a history of miscarriage or infertility may have subclinical hypothyroidism.
Beyond the hormone levels themselves, the presence of thyroid autoantibodies, particularly Thyroid Peroxidase Antibodies (TPOAb), independently increases the risk of miscarriage. TPOAb are associated with Hashimoto’s thyroiditis, the most common cause of hypothyroidism in iodine-sufficient regions. Women who are TPOAb-positive have a higher risk of pregnancy loss, even if their TSH and Free T4 levels are currently considered normal.
Diagnosis and Management Strategies to Mitigate Risk
Effective management begins with appropriate screening to identify women at risk, which can happen either before conception or at the first prenatal visit. Standard screening involves measuring the TSH level in the blood. If a woman has a history of miscarriage, infertility, or other risk factors, TPOAb testing is also performed.
The goal for thyroid function during pregnancy is to maintain TSH levels within a tighter range than for non-pregnant adults. Current guidelines recommend a first-trimester TSH target of below 2.5 mIU/L, and a TSH above this level may prompt intervention. The standard treatment for hypothyroidism in pregnancy is the daily administration of Levothyroxine (LT4), a synthetic form of the T4 hormone.
Pregnant women with a pre-existing diagnosis of hypothyroidism usually require an increase in their Levothyroxine dosage, often by 20% to 30%, as early as the first weeks of pregnancy. This adjustment is necessary to meet the higher metabolic demands of gestation. To ensure the TSH target is maintained, thyroid function tests must be monitored regularly, every four to six weeks throughout the entire pregnancy. Treatment with Levothyroxine substantially reduces the risk of miscarriage in women with overt and subclinical hypothyroidism, as well as in those who are euthyroid but TPOAb-positive.