hypothyroidism during pregnancy

Pathology

• increased need for thyroxine during pregnancy, 30-50% higher on average
• increased plasma thyroid-binding globulin
• changes in peripheral metabolism of thyroid hormones
• increased dietary iodine requirement
• transient stimulation of the maternal thyroid by chorionic gonadotropin

Laboratory

• serum TSH as soon as pregnancy test is positive & every 4 weeks during the 1st 1/2 of pregnancy, then at least once between 26-32 weeks of gestation
  • if serum TSH is > 2.5 uIU/mL in 1st trimester or 3-10 uU/mL in 2nd & 3rd trimesters, measure free T4 in serum
  • if serum TSH > 10 uIU/mL, initiate therapy with levothyroxine
• serum TSF 6 weeks after delivery & return of levothyroxine dose to pre-pregnancy dose
• thyroid peroxidase Ab in serum
  • if positive & serum TSH > 2.5 uIU/mL in 1st trimester or > 3.0 uIU/mL in 2nd or 3rd trimesters, initiate therapy with levothyroxine

Complications

• increased risk of
  • miscarriage
  • preterm bith, low birthweight
  • cognitive impairment (infant & Mom)

Management

• begin treatment before a woman becomes pregnant
• titrate levothyroxine doses to achieve a serum TSH < 2.5 mIU/L when a woman is planning to become pregnant
• as soon as pregnancy is confirmed, increase the levothyroxine dose by 25-30%
  • extra dose of levothyroxine 2 days/week as soon as she misses a period or has a positive home pregnancy test
• if Hashimoto's thyroiditis, titrate levothyroxine to achieve serum TSH target range
• serum TSH target ranges:
  • < 2.5 mIU/L 1st trimester
  • < 3.0 mIU/L 2nd & 3rd trimesters
• after delivery, resume pre-pregnancy levothyroxine dose
• separate levothyroxine & prenatal vitamins
  • take levothyroxine first thing in the morning on an empty stomach
  • take prenatal vitamin at least 4 hours later with lunch or dinner

More general terms

• hypothyroidism
• pregnancy disorder; obstetric disorder; pregnancy complication