Hypothyroidism in Pregnancy

Aka: Hypothyroidism in Pregnancy

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II. Epidemiology

  1. Hypothyroidism Incidence in pregnancy (U.S.)
    1. Overt Hypothyroidism: 0.3 to 0.5% of pregnancies
    2. Subclinical Hypothyroidism: 2-3% of pregnancies

III. Precautions

  1. Maintaining euthyroid state in pregnancy is critical (see complications below)
  2. Endocrinology consult is recommended
  3. Avoid desiccated Thyroid (e.g. armour Thyroid) or Liothyronine (T3) in pregnancy
    1. Inadequate T4 for the fetus

IV. Findings

  1. See Hypothyroidism

V. Indications: Screening

  1. See Thyroid Dysfunction in Pregnancy

VI. Labs: Serum TSH Monitoring protocol

  1. Initial testing
    1. Serum TSH at earliest pregnancy diagnosis (Levothyroxine increased at this time)
    2. Serum TSH 4-6 weeks after initial Levothyroxine increase (and then per protocol below)
  2. Subsequent testing in pregnancy
    1. Serum TSH every 4-6 weeks until 20 weeks gestation AND dose stable, then
    2. Serum TSH at 24-28 weeks and 32-34 weeks
  3. Perinatal and postpartum TSH
    1. Serum TSH at perinatal period is not required unless otherwise indicated
    2. Serum TSH at 4-6 weeks after delivery (levoothyroxine dose decreased to baseline after delivery)

VII. Management: Clinical Hypothyroidism onset in Pregnancy

  1. Clinical Hypothyroidism (esp. Hashimoto Thyroiditis) complicates 2 per 1000 pregnancies
  2. Start Levothyroxine and titrate based on every 4-6 week TSH monitoring

VIII. Management: Levothyroxine increased dose at onset of pregnancy

  1. Pregnancy requires an increased Levothyroxine dose
    1. See Thyroid Dysfunction in Pregnancy
    2. Increased dose by 30-47% over baseline required in most pregnant patients
    3. Increased dose required for remainder of pregnancy (with frequent monitoring of TSH)
  2. Increase Levothyroxine dose at earliest knowledge of pregnancy
    1. Achieve euthyroid state as soon as possible
    2. Recheck Serum TSH 4-6 weeks (30-40 days) after Levothyroxine dose start and dose change
    3. Educate patient to take Levothyroxine at consistent time and preferably 4 hours apart from PNV
  3. Add 2 additional doses per week (9 total doses)
    1. After a first missed menstrual period or positive Pregnancy Test
      1. Patient adds 2 additional Levothyroxine doses per week AND
      2. Notify treating medical provider
    2. Example
      1. Patient on 100 mcg daily before pregnancy
      2. When pregnancy diagnosed, start taking an extra dose (total 200 mcg) on Tuesday and Saturday
      3. Recheck Thyroid Stimulating Hormone (TSH) at 4-6 weeks after dose change
    3. Efficacy
      1. Dose adjustment safely and completely prevents TSH increase above 5.0 mIU/L
      2. Prevents >2.5 mIU/L in 85% of cases
      3. Yassa (2010) J Clin Endocrinol Metab 95(7): 3234-41 [PubMed]

IX. Management: Levothyroxine dose adjustment during pregnancy

  1. Initiate Levothyroxine dosing protocol following the initial increase in Levothyroxine dose at pregnancy diagnosis
  2. Goal Thyroid Stimulating Hormone
    1. Goal TSH <2.5 mcg/day preconception and first trimester
    2. Goal TSH <3.0 for second and third trimester
    3. Abalovich (2007) J Clin Endocrinol Metab 92(8 Suppl):S1-47 [PubMed]
  3. Thyroid Stimulating Hormone (TSH): 2.5-5.0 mIU/L (or 3.0 to 5.0 after first trimester)
    1. Increase daily Levothyroxine dose by 12.5-25 mcg/day
  4. Thyroid Stimulating Hormone (TSH): 5-10 mIU/L
    1. Increase daily Levothyroxine dose by 25-50 mcg/day
  5. Thyroid Stimulating Hormone (TSH): 10-20 mIU/L
    1. Increase daily Levothyroxine dose by 50-75 mcg/day
  6. Thyroid Stimulating Hormone (TSH): >20 mIU/L
    1. Increase daily Levothyroxine dose by 75-100 mcg/day

X. Management: Postpartum Levothyroxine dosing

  1. Gradually decrease dose to baseline (pre-pregnant dose) over first 4 weeks after delivery
  2. Recheck Serum TSH at 4-6 weeks after delivery

XI. Complications

  1. Decreased with Thyroid Replacement (Levothyroxine)
    1. Miscarriage
    2. Preterm Birth
    3. Fetal cognitive deficits
  2. Not affected by Thyroid Replacement
    1. Hypertensive Disorders of Pregnancy (including Preeclampsia)
    2. Low birth weight or Preterm birth
    3. Placental Abruption

XII. References

  1. (2018) Presc Lett 25(11): 65
  2. Carney (2014) Am Fam Physician 89(4): 273-8 [PubMed]
  3. De Groot (2012) J Clin Endocrinol Metab 97(8): 2543-65 [PubMed]
  4. Stagnaro-Green (2011) Thyroid 21(10): 1081-125 [PubMed]
  5. Wilson (2021) Am Fam Physician 103(10): 605-13 [PubMed]

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