12- PREGNANCY AND HYPO/HYPERTHYROIDISM

• Uncontrolled hypothyroid mother

  Impaired fetal neuro-cognitive development, increased risks of premature birth, low birth weight, pregnancy loss, and lower offspring IQ (cretinism)
• Uncontrolled hyperthyroid mother

  Pregnancy loss, pregnancy-induced HTN, prematurity, low birth weight, intrauterine growth restriction, stillbirth, thyroid storm, and maternal congestive heart failure
• Thyroxine levels in pregnancy

  1. hCG is similar to TSH à stimulates the thyroid à more thyroid hormones

  2. Increased estrogen produces higher levels of thyroid-binding globulin à decreases the amount of free thyroid hormones in the blood à contributes to hypothyroidism and required higher replacement dose
• Goal of thyroid hormone treatment

  To normalize maternal serum TSH values within the trimester-specific pregnancy reference range
• Trimester-specific pregnancy reference range for TSH

  • First trimester, 0.1-2.5 mIU/L
  • Second trimester, 0.2-3.0 mIU/L
  • Third trimester, 0.3-3.0 mIU/L
• Thyroid hormone adjustments in pregnancy

  1. At 4-6 weeks of gestation a dosage increase of 25-30% may be needed because the increased requirement for thyroid hormone (endogenous or exogenous thyroxine) occurs as early as 4–6 weeks of pregnancy & increases gradually through 16-20 weeks of pregnancy, and thereafter plateaus until time of delivery

  2. After delivery, the dosage of thyroid medication should be reduced to the preconception dose. TSH testing should be performed at 6 weeks postpartum
• Grave's disease and pregnancy

  • Fetal or neonatal hyperthyroidism (from high levels of thyroid stimulating antibodies (TRAb) in the second half of pregnancy)
  • Fetal or neonatal hypothyroidism (from excessive amounts of anti-thyroid drugs even if the mother is biochemically euthyroid)
  • Central hypothyroidism
• TRAb

  Thyroid stimulating antibodies, measurable in around 95% of patients with active Graves' disease, and levels may still remain high following ablation therapy
• Measuring TRAb in pregnancy

  1. In early pregnancy: to detect pregnancies at risk (If >5 IU/L or 3 times the upper limit of normal in mother à close follow-up of fetal heart rate)

  2. In late pregnancy: to detecting risk for neonatal hyperthyroidism (If >5 IU/L or 3 x upper limit of normal à neonate may become hyperthyroid)

  3. If TRAb becomes undetectable in a pregnant woman taking antithyroid drugs, it may be feasible to reduce or withdraw the drugs to protect the fetus against hypothyroidism and goiter
• Anti-thyroid drug use in pregnancy

  PTU must be used in the first trimester and then switched to methimazole in the second and third trimesters. Both drugs can cross the placenta causing hypothyroidism, which causes mental retardation; therefore, the lowest dose must be used