What is the treatment for subclinical hyperthyroidism in pregnancy?

Management of Subclinical Hyperthyroidism in Pregnancy

For subclinical hyperthyroidism in pregnancy, close monitoring is generally recommended rather than immediate treatment, unless there are clinical signs of hyperthyroidism or other risk factors present. 1

Diagnosis and Evaluation

• Subclinical hyperthyroidism is characterized by low or suppressed TSH with normal Free T4 or Free T4 Index (FTI) levels 1
• Complete evaluation should include testing for TSH and Free T4 or FTI to confirm the diagnosis 1
• Distinguish between subclinical hyperthyroidism and hCG-mediated thyroid stimulation, which is common in early pregnancy and rarely requires treatment 1, 2

Treatment Approach

When to Monitor Without Treatment

• Subclinical hyperthyroidism without clinical symptoms often requires only monitoring rather than active treatment 3
• HCG-induced subclinical hyperthyroidism typically resolves spontaneously and requires only periodic thyroid function monitoring 2, 3
• Several thyroid function controls should be performed to determine if treatment is necessary in cases of subclinical hyperthyroidism 3

When to Consider Treatment

• Treatment should be considered if:
  • Clinical symptoms of hyperthyroidism are present 1
  • There is evidence of fetal compromise 1
  • The condition is due to Graves' disease rather than physiologic changes of pregnancy 4
  • The subclinical hyperthyroidism is severe (TSH completely suppressed) 5

Medication Management When Treatment is Indicated

• Propylthiouracil (PTU) is the preferred medication during the first trimester 6, 1
• Methimazole (MMI) is preferred in the second and third trimesters due to lower risk of hepatotoxicity 6, 1
• The goal of treatment is to maintain Free T4 or FTI in the high-normal range using the lowest possible thioamide dosage 1, 4
• Treatment should aim for a state of mild subclinical hyperthyroidism rather than complete normalization 4

Monitoring During Pregnancy

• Monitor Free T4 or FTI every 2-4 weeks initially to adjust medication dosage if treatment is initiated 1
• Once stable, check thyroid function every trimester 1
• Women with Graves' disease should be monitored for normal fetal heart rate and appropriate growth 1

Risks of Untreated Significant Hyperthyroidism

• Increased risk of severe preeclampsia 1
• Higher rates of preterm delivery 1, 4
• Potential for maternal heart failure 1
• Increased risk of miscarriage 1, 4
• Low birth weight infants 1, 4

Medication Considerations and Cautions

• Antithyroid drugs cross the placenta and can affect fetal thyroid function 3, 4
• PTU has been associated with hepatotoxicity, particularly beyond the first trimester 6
• Methimazole has been associated with rare congenital anomalies when used in the first trimester 6, 7
• The lowest effective dose should be used to minimize fetal exposure 1, 4

Postpartum Considerations

• Women treated with either propylthiouracil or methimazole can safely breastfeed 1
• Hyperthyroidism frequently relapses postpartum (reported in up to 83% of Graves' disease patients) 4
• Monitor for postpartum thyroiditis in women with history of thyroid dysfunction 1