What is the management of uncontrolled hypothyroidism (underactive thyroid) during pregnancy to minimize impact on the fetus?

Management of Uncontrolled Hypothyroidism During Pregnancy to Minimize Fetal Impact

Uncontrolled hypothyroidism during pregnancy must be treated immediately with levothyroxine to normalize TSH levels and prevent serious adverse effects on fetal development including growth failure, mental retardation, and neuropsychological defects. 1

Risks of Uncontrolled Hypothyroidism in Pregnancy

• Untreated maternal hypothyroidism increases the risk of preeclampsia, preterm delivery, and low birth weight in neonates 1
• Maternal hypothyroidism from iodine deficiency increases the risk of congenital cretinism (growth failure, mental retardation, other neuropsychologic defects) 1
• Inadequate thyroid hormone availability negatively impacts fetal brain development, especially during the first trimester when the fetal thyroid is not yet functional 2
• Even subclinical hypothyroidism may lead to adverse gestational outcomes and intellectual impairment of offspring 3

Diagnostic Approach

• TSH testing is the recommended initial test for screening and evaluating thyroid function in pregnancy 1
• TSH and Free T4 (FT4) or Free Thyroxine Index (FTI) testing should be performed in pregnant women with suspected hypothyroidism 1
• Subclinical hypothyroidism is identified by an elevated TSH level in a pregnant woman without symptoms 1

Treatment Protocol

Initial Treatment

• Start levothyroxine immediately upon diagnosis of hypothyroidism during pregnancy 4, 3
• For women with pre-existing hypothyroidism who become pregnant:
  • Increase levothyroxine dose by approximately 30% as soon as pregnancy is confirmed 5
  • The mean levothyroxine requirement increases by 47% during the first half of pregnancy, with onset of increase typically around 8 weeks gestation 5

Dosage Adjustment

• Adjust levothyroxine dosage every four weeks until the TSH level is stable 1
• Target TSH should be in the normal reference range 1
• For women already on levothyroxine with subclinical hypothyroidism, assess compliance and appropriateness of dose 1
• The increased dose requirement typically plateaus by week 16 of pregnancy and remains elevated until delivery 5

Monitoring Protocol

• Check TSH level every trimester at minimum 1
• More frequent monitoring (every 6-8 weeks) is recommended to ensure adequate thyroid hormone replacement 1
• Monitor for normal fetal heart rate and appropriate growth 1

Special Considerations

• Levothyroxine requirements often increase as early as the fifth week of gestation 5
• Women with Hashimoto's disease (most common cause in industrialized nations) may require more aggressive monitoring due to the autoimmune nature of the disease 1
• Iodine therapy in the first and second trimesters can significantly reduce neurologic abnormalities in cases of iodine deficiency 1

Post-Delivery Management

• Levothyroxine dosage should return to pre-pregnancy dose immediately after delivery 4
• Continue monitoring thyroid function postpartum as requirements typically decrease 6

Potential Medication Interactions

• Certain foods may affect levothyroxine absorption:
  • Soybean flour, cottonseed meal, walnuts, and dietary fiber may bind and decrease absorption 4
  • Grapefruit juice may delay absorption and reduce bioavailability 4
• Concurrent medications may interact with levothyroxine:
  • Oral anticoagulants (increased response, may need dose reduction) 4
  • Digitalis glycosides (reduced therapeutic effects) 4
  • Antidepressants (may increase therapeutic and toxic effects) 4

Pitfalls to Avoid

• Delaying treatment - even mild hypothyroidism can have significant adverse effects on fetal development 2
• Inadequate dose adjustment - requirements increase early in pregnancy and failure to increase dosage promptly can result in persistent hypothyroidism 5
• Insufficient monitoring - TSH levels should be checked regularly throughout pregnancy 1
• Radioactive iodine (I-131) is absolutely contraindicated during pregnancy and lactation 7