The thyroid gland produces hormones that regulate the body’s energy use and metabolism. Thyroid-Stimulating Hormone (TSH) is a pituitary hormone that signals the thyroid how much hormone to produce. Maintaining proper thyroid hormone levels is significant for a healthy pregnancy because these hormones are directly involved in the baby’s brain and nervous system development. The fetus relies entirely on the mother’s thyroid hormone supply for the first 18 to 20 weeks of gestation.
How Pregnancy Alters Thyroid Function
The hormonal environment of pregnancy dramatically changes how the thyroid gland operates. Two main hormones drive these changes: human chorionic gonadotropin (hCG) and estrogen. hCG, the hormone detected by pregnancy tests, shares a structural similarity with TSH and weakly stimulates the thyroid gland to produce more hormone.
High hCG levels, which peak in the first trimester, temporarily suppress the pituitary’s production of TSH, leading to lower TSH levels in the mother’s blood. Simultaneously, increased estrogen levels cause a rise in thyroid-binding globulin (TBG), a protein that transports thyroid hormones. This increase in TBG means the thyroid must produce about 50% more thyroid hormone to maintain an adequate supply of the active, or “free,” hormone available to the mother and fetus.
The mother must also increase iodine intake because iodine is a required component of thyroid hormones. The developing fetus requires a constant supply of these hormones to support its growth, particularly its brain. The substantial physiological demand on the maternal thyroid often acts as a “stress test,” revealing underlying thyroid conditions in women with limited thyroid reserve.
Recommended TSH Targets for Each Trimester
Because of these physiological changes, TSH target ranges during pregnancy are significantly lower than for non-pregnant adults. Healthcare providers rely on trimester-specific reference intervals to accurately assess thyroid function. Guidelines from organizations like the American Thyroid Association (ATA) recommend distinct upper limits for TSH in each stage of gestation.
In the first trimester, when hCG levels are highest and TSH is naturally suppressed, the upper limit for TSH is targeted to be below 2.5 mIU/L. This low target reflects the period of peak fetal reliance on maternal hormone supply. The TSH level often stabilizes in the second trimester, and the target upper limit increases slightly to below 3.0 mIU/L.
For the third trimester, the target upper limit for TSH remains below 3.0 mIU/L, consistent with the second trimester range. Laboratories should establish their own population-based, trimester-specific reference ranges. If a lab has not established its own range, the 2.5 mIU/L and 3.0 mIU/L cutoffs are the standard limits for initiating treatment in women with elevated TSH.
Potential Risks of Abnormal TSH Levels
Deviations from the recommended TSH targets can signal thyroid dysfunction, posing risks to both the mother and the developing baby. High TSH levels, which indicate an underactive thyroid (hypothyroidism), are a concern. Untreated maternal hypothyroidism is linked to adverse maternal outcomes, including an increased risk of preeclampsia and gestational hypertension.
Fetal and neonatal risks from high TSH levels can be severe, especially if the condition is overt. Untreated hypothyroidism is associated with preterm birth, low birth weight, and placental abruption. Most concerning is impaired neuropsychological and cognitive development in the child due to insufficient thyroid hormone supply during the first half of pregnancy. Even mildly elevated TSH levels, known as subclinical hypothyroidism, have been associated with increased risks of prematurity and neonatal respiratory distress syndrome.
Conversely, low TSH levels, indicating an overactive thyroid (hyperthyroidism), also carry risks. Severe hyperthyroidism can lead to maternal complications such as congestive heart failure and preeclampsia. For the fetus, untreated hyperthyroidism is associated with premature birth and low birth weight.
Monitoring and Treatment Protocols
A targeted screening approach is often used, where TSH testing is performed on women with risk factors such as a history of thyroid disease or positive thyroid antibodies. If a pregnant woman is already being treated for hypothyroidism, her levothyroxine dose will need to be increased to meet the heightened demands of pregnancy. Many guidelines suggest that women with known hypothyroidism should increase their dose by 25% to 30% immediately upon confirmation of pregnancy.
For women diagnosed with hypothyroidism during pregnancy, the primary treatment is synthetic thyroxine, or levothyroxine. The goal is to normalize the TSH level into the trimester-specific target range, aiming for a TSH below 2.5 mIU/L. TSH levels are monitored frequently, typically every four to six weeks, until mid-gestation and until a stable medication dose is achieved.
Management of hyperthyroidism (low TSH) involves anti-thyroid medications, such as propylthiouracil in the first trimester, to lower thyroid hormone production. The aim is to maintain free thyroxine levels in the upper third of the normal range. Close monitoring and careful dose adjustment are necessary for both high and low TSH conditions to ensure the best outcomes for both mother and baby.