What are the guidelines for thyroid function testing in a pregnant female with a history of thyroid disease in her 2nd trimester?

Thyroid Testing in Second Trimester for Pregnant Women with History of Thyroid Disease

Testing Recommendations

For pregnant women with a history of thyroid disease, measure TSH and free T4 (FT4) every 4 weeks during the second trimester until stable, then continue monitoring once per trimester. 1, 2

Initial Assessment Strategy

• Screen all pregnant women with a history of thyroid disease - this is non-negotiable regardless of symptoms 1, 2
• Obtain both TSH and FT4 or free thyroxine index (FTI) together as the initial test panel 1
• Do not rely on TSH alone in pregnancy, as physiologic changes can suppress TSH even in euthyroid states 3, 4

Monitoring Frequency Based on Condition

For Pre-existing Hypothyroidism on Levothyroxine:

• Check TSH and FT4 every 4 weeks after any dosage adjustment 5
• Once stable, check TSH at minimum once per trimester 1, 5
• Increase levothyroxine dose by 12.5-25 mcg if TSH rises above trimester-specific reference range 5
• Most women require a 30-50% increase in levothyroxine dosage during pregnancy 5

For Pre-existing Hyperthyroidism or Graves' Disease:

• Monitor FT4 or FTI every 2-4 weeks during active treatment until stable 1, 6, 2
• Goal is to maintain FT4 in the high-normal range using the lowest possible thioamide dose 1, 6, 2
• Continue monitoring fetal heart rate and growth throughout pregnancy 1, 6

Critical Interpretation Considerations

• Use trimester-specific reference ranges - standard non-pregnant reference ranges will lead to misdiagnosis 3
• TSH normally decreases in first trimester due to hCG stimulation, then gradually rises 3, 4
• FT4 gradually decreases throughout pregnancy due to increased thyroxine-binding globulin 3, 4
• Total T4 increases substantially during pregnancy and should not be used for monitoring 4

Common Pitfalls to Avoid

• Do not skip testing asymptomatic women with thyroid history - targeted high-risk screening misses 30% of cases with thyroid dysfunction 7
• Do not wait for symptoms to develop before testing, as many pregnancy symptoms overlap with thyroid dysfunction 1
• Do not use total thyroid hormone measurements - only free T4 or FTI are reliable in pregnancy 1, 4
• Do not assume stable pre-pregnancy thyroid function will remain stable - pregnancy dramatically alters thyroid hormone requirements 5, 3

Additional Testing When Indicated

• Consider thyroid peroxidase antibodies (TPOAb) if not previously measured, as positive antibodies increase risk of postpartum thyroiditis 1
• If hyperthyroidism is detected, measure TSH receptor antibodies to distinguish Graves' disease from gestational transient thyrotoxicosis 6
• Gestational transient thyrotoxicosis (associated with hyperemesis gravidarum) rarely requires treatment and resolves spontaneously 1, 6