What are the thyroid gland changes during pregnancy?

From the Guidelines

During pregnancy, it is crucial to monitor and manage thyroid function to prevent adverse outcomes, as uncontrolled thyroid disease can lead to significant maternal and neonatal morbidity, and proper thyroid function affects both maternal health and fetal development, particularly neurological development in the first trimester. The thyroid gland undergoes significant physiological changes to meet increased metabolic demands, typically enlarging by 10-15% in iodine-sufficient areas and up to 20-40% in iodine-deficient regions, with total thyroid hormone production increasing by approximately 50% during pregnancy, primarily due to elevated estrogen levels which increase thyroxine-binding globulin (TBG) concentrations, as noted in studies 1.

Some key points to consider in the management of thyroid disease during pregnancy include:

• Women with pre-existing thyroid conditions often need medication adjustments during pregnancy, with those with hypothyroidism typically requiring a 30-50% increase in levothyroxine dosage as early as the first trimester, as recommended by guidelines 1.
• For women with hyperthyroidism, medications like propylthiouracil are preferred in the first trimester, while methimazole may be used in later pregnancy, due to the potential risks associated with each medication, as discussed in studies 1.
• Adequate iodine intake (250 mcg daily) is essential during pregnancy to support these increased demands, and routine screening for subclinical hypothyroidism is not recommended, but women with risk factors and symptoms of thyroid diseases should be screened, and subclinical hypothyroidism should be treated, as suggested by research 1.
• The goal of treatment for hyperthyroidism is to maintain FT4 or FTI in the high-normal range using the lowest possible thioamide dosage, with monitoring every two to four weeks, and treatment with iodine 131 (I-131) is contraindicated in pregnant women, as stated in guidelines 1.

Overall, proper management of thyroid function during pregnancy is critical to prevent adverse outcomes and ensure the best possible health for both mother and fetus, and healthcare providers should be aware of the potential risks and benefits associated with different treatment options, as discussed in studies 1.

From the FDA Drug Label

Pregnancy may increase levothyroxine sodium requirements. Serum TSH levels should be monitored and the levothyroxine sodium dosage adjusted during pregnancy Maternal hypothyroidism during pregnancy is associated with a higher rate of complications, including spontaneous abortion, gestational hypertension, pre‑eclampsia, stillbirth, and premature delivery. Untreated maternal hypothyroidism may have an adverse effect on fetal neurocognitive development Since TSH levels may increase during pregnancy, TSH should be monitored and levothyroxine sodium dosage adjusted during pregnancy

Thyroid changes during pregnancy may include increased requirements for levothyroxine sodium due to changes in TSH levels.

• Key points to consider:
  • Monitor serum TSH levels during pregnancy
  • Adjust levothyroxine sodium dosage as needed
  • Untreated maternal hypothyroidism can have adverse effects on fetal development and increase the risk of pregnancy complications 2

From the Research

Thyroid Changes During Pregnancy

Thyroid changes during pregnancy can have significant health implications for both the mother and the fetus.

• Thyroid disease is the second most common endocrine disorder affecting women of reproductive age, and when untreated during pregnancy, it is associated with an increased risk of miscarriage, placental abruption, hypertensive disorders, and growth restriction 3.
• Current guidelines recommend targeted screening of women at high risk, including those with a history of thyroid disease, type 1 diabetes mellitus, or other autoimmune disease; current or past use of thyroid therapy; or a family history of autoimmune thyroid disease 3.
• Appropriate management results in improved outcomes, demonstrating the importance of proper diagnosis and treatment 3.

Hypothyroidism During Pregnancy

• In women with hypothyroidism, levothyroxine is titrated to achieve a goal serum thyroid-stimulating hormone level less than 2.5 mIU per L 3.
• The drug of choice in substitution therapy of hypothyroidism is levothyroxine 4.
• Treatment of subclinical hypothyroidism is generally not necessary unless the TSH exceeds 7.0-10 mIU/L 5.

Hyperthyroidism During Pregnancy

• The preferred treatment for hyperthyroidism is antithyroid medications, with a goal of maintaining a serum free thyroxine level in the upper one-third of the normal range 3.
• Administration of propylthiouracil should be limited to the first trimester of pregnancy, because its serious hepatotoxicity has been described 4.
• Hyperthyroidism based on thyroid nodules and immunogenic hyperthyroidism not reaching long-term remission, need to be treated radically: by surgery or radioiodine treatment 4.

Postpartum Thyroiditis

• Postpartum thyroiditis is the most common form of postpartum thyroid dysfunction and may present as hyper- or hypothyroidism 3.
• Symptomatic treatment is recommended for hyperthyroidism; levothyroxine is indicated for hypothyroidism in women who are symptomatic, breastfeeding, or who wish to become pregnant 3.