Harrison's PodClass: Internal Medicine Cases and Board Prep Ep 164: A 25-Year-Old Pregnant Woman Asking About Her Thyroid
Sep 18, 2025
Explore the fascinating dynamics of thyroid function during pregnancy with a real-life case involving a 25-year-old patient. Discover how TRH, TSH, and T4/T3 interact, along with factors that can alter thyroid tests. Learn why universal screening isn't recommended and who should undergo targeted TSH testing. The discussion also highlights the increased need for thyroid replacement during pregnancy, paving the way for crucial clinical insights.
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Pregnancy Redefines Thyroid Tests
- Pregnancy alters thyroid physiology via hCG, increased TBG, immune changes, placental metabolism, and iodine loss.
- These shifts raise total T3/T4 and lower TSH in early pregnancy without large free-hormone changes.
TSH Is A Reliable Single-Test Screen
- TSH is the set point of the thyroid axis and single TSH measurements are clinically adequate despite pulsatility.
- Low TSH indicates primary hyperthyroidism; high TSH indicates primary hypothyroidism.
Ensure Adequate Iodine Intake
- Ensure adequate iodine intake in pregnancy: WHO recommends ~250 μg/day and prenatal vitamins ~150 μg/tablet.
- Consider iodine especially in regions with marginal iodine sufficiency to prevent maternal and fetal thyroid issues.
