What is the approach to managing hypothyroidism (underactive thyroid) in a pregnant woman?

Management of Hypothyroidism in Pregnancy

Pregnant women with hypothyroidism should be treated immediately with levothyroxine to normalize TSH levels, with dosage adjustments every 4 weeks until stable, then monitoring each trimester. 1

Initial Diagnosis and Assessment

TSH testing is the recommended initial screening test for evaluating thyroid function in pregnant women with suspected hypothyroidism. 1 When hypothyroidism is suspected, both TSH and free T4 (or free thyroxine index) should be measured to confirm the diagnosis and determine severity. 1

Who Should Be Screened

Screening is appropriate for pregnant women with: 1

• Family or personal history of thyroid disease
• Physical findings or symptoms suggestive of goiter or hypothyroidism
• Type 1 diabetes mellitus
• Personal history of autoimmune disorders

Universal screening of all pregnant women is not recommended by most professional groups due to inconsistent evidence regarding benefits. 2

Treatment Protocol

Overt Hypothyroidism

Initiate levothyroxine immediately in sufficient dosage to return TSH to normal range. 1 The goal is to achieve euthyroidism as rapidly as possible to prevent adverse maternal and fetal outcomes. 2

For women with pre-existing hypothyroidism on levothyroxine: 3

• Increase the levothyroxine dose by 12.5 to 25 mcg per day when TSH is above the normal trimester-specific range
• Alternatively, increase the pre-pregnancy dose by approximately 30% as soon as pregnancy is confirmed 4
• Monitor TSH every 4 weeks until stable and within normal trimester-specific range

For new-onset overt hypothyroidism (TSH ≥10 mIU/L): 3

• Start levothyroxine at 1.6 mcg/kg/day
• Monitor TSH every 4 weeks and adjust dosage until TSH is within normal trimester-specific range

For new-onset mild hypothyroidism (TSH <10 mIU/L): 3

• Start levothyroxine at 1.0 mcg/kg/day
• Monitor and adjust similarly

Subclinical Hypothyroidism

Pregnant women with elevated TSH should be treated with levothyroxine to restore TSH to the reference range, based on the possible association between elevated TSH and increased fetal wastage or neuropsychological complications in offspring. 1 This recommendation applies even though there are no published intervention trials, as the potential benefit-risk ratio justifies treatment. 1

Monitoring Strategy

During Pregnancy

Adjust levothyroxine dosage every 4 weeks until TSH is stable. 1 Once stable, check TSH every trimester. 1

For women already on levothyroxine, TSH should be monitored every 6 to 8 weeks during pregnancy according to some guidelines 1, though the ACOG guideline suggests every 4 weeks until stable. 1

More than 50% of women with pre-existing hypothyroidism require dose increases during pregnancy. 2 The levothyroxine requirement typically increases by 47% during the first half of pregnancy, with the median onset at 8 weeks gestation, plateauing by week 16. 4

Timing of Dose Adjustments

The need for increased levothyroxine can appear as early as 4-8 weeks of gestation or as late as the third trimester. 5 Evidence suggests that increasing levothyroxine by two additional tablets per week (approximately 30% increase) immediately upon pregnancy confirmation prevents maternal hypothyroidism during the first trimester. 6, 4

Postpartum Management

Reduce levothyroxine dosage to pre-pregnancy levels immediately after delivery. 3 Monitor serum TSH 4 to 8 weeks postpartum to ensure appropriate dosing. 3

Critical Risks of Untreated Hypothyroidism

Untreated or inadequately treated hypothyroidism during pregnancy is associated with: 7, 8

• Increased risk of preeclampsia
• Low birth weight
• Impaired fetal neuropsychological development
• Increased fetal mortality and miscarriage 2, 8
• Congenital cretinism (particularly with iodine deficiency) 1, 7

Common Pitfalls to Avoid

Failure to increase levothyroxine dose early in pregnancy is a common error. 4 Real-world evidence suggests hypothyroidism in pregnancy is often overlooked or levothyroxine is not appropriately dosed to achieve tight TSH control. 8

Do not wait for symptoms to develop before adjusting dosage - biochemical monitoring should guide therapy. 1

For women with pre-existing hypothyroidism, measure TSH and free T4 as soon as pregnancy is confirmed and maintain TSH in the trimester-specific reference range throughout pregnancy. 3

Patients at higher risk for TSH suppression with early dose increases include those who are athyreotic, have prepregnancy TSH <1.5 mIU/L, or receive prepregnancy levothyroxine doses ≥100 mcg/day. 6 These patients may require more conservative initial dose adjustments.