II. Physiology: Pregnancy specific Thyroid changes

  1. Thyroid Hormone requirements increase by 20-40% in pregnancy (as early as 4 weeks gestation)
    1. Thyroid Binding Globulin increases (Estrogen mediated)
    2. Thyroid Hormone (Thyroxine) has a greater volume of distribution
    3. Placental metabolism of Thyroxine
    4. HCG is a weak TSH receptor stimulator in early pregnancy (decreases TSH levels), but normalizes by second trimester
  2. Thyroid function markers are less reliable in pregnancy
    1. Serum TSH normal range is lower in pregnancy (due to cross-reaction with aHCG)
    2. Serum T4 Free is affected by serum binding Protein level changes

III. Indications: Thyroid testing in pregnancy

  1. Many Prenatal Lab testing protocols routinely include Thyroid Stimulating Hormone (TSH)
  2. Current or prior Thyroid disorder management
    1. Hypothyroidism
    2. Hyperthyroidism
    3. Thyroid Goiter
    4. Postpartum Thyroid dysfunction history
    5. TPO Antibody Positive
  3. Family History of autoimmune Thyroid disease (e.g. Grave's Disease, Hashimoto's Thyroiditis)
  4. Type I Diabetes Mellitus
  5. Autoimmune disorder history
  6. High dose neck radiation exposure history
  7. Neontal Hypothyroidism with prior delivery

IV. Labs: Thyroid Testing

  1. Serum Thyroid Stimulating Hormone (TSH) normal ranges
    1. Non-pregnanct: 0.3 to 4.3 mIU/ml
    2. First trimester: 0.1 to 2.5 mIU/ml
    3. Second trimester: 0.2 to 3.0 mIU/ml
    4. Third timester: 0.3 to 3.0 mIU/ml
  2. Free Thyroxine (Free T4) normal ranges
    1. Non-pregnant: 0.8 to 1.7 ng/dl
    2. First trimester: 0.8 to 1.2 ng/dl
    3. Second trimester: 0.6 to 1.0 ng/dl
    4. Third timester: 0.5 to 0.8 ng/dl
  3. References
    1. Abbassi (2010) Obstet Gynecol 114(6): 1326-31 [PubMed]

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