Thyroid Disease in Pregnancy

Summary of American Thyroid Association Guideline 2011

Hypothyroidism

Gestational age specific TSH reference ranges, from Obstet Gynecol 2005 106 753

TSH targets

Preferably, laboratory specific reference ranges. If not available:

  • 1st trimester: 0.1-2.5
  • 2nd trimester: 0.2-3.0
  • 3rd trimester: 0.3-3.0

Overt Hypothyroidism

  • Defined as TSH >10, or T4 below range.
  • Definitely treat. Goal is to normalize TSH to trimester-specific reference range.

Treated Hypothyroidism

  • Increase dose by 25-30% after missed period or positive test.
  • Monitor TFTs every 4 weeks up to 20/40, and at least once at 26-32/40.

Subclinical Hypothyroidism

  • No guideline whether to treat (note Endo Soc guidelines 2007 did advise treatment).
  • Definitely monitor TFTs during pregnancy. Every 4 weeks until 20/40, and at least once at 26-32/40.

Hypothyroxinaemia

  • No treatment (see Lazarus article).

Anti-TPO Positivity

  • Increases risk of hypothyroidism to around 20%.
  • If known TPO positive, monitor TFTs every 4 weeks until 20/40, and at least once at 25-32/40.
  • Is linked to miscarriage, but insufficient evidence to justify treatment. Small studies purport to show a benefit of T4 replacement.

Hyperthyroidism

Prior History of Graves' disease and Pregnant

  • Measure TRAb at 20-24/40.
  • Monitor fetus closely if TRAb>3xULN, or maternal thyrotoxicosis.

Gestational Hyperthyroidism

  • Differentiate from Graves' disease. In gestational hyperthyroidism, there is physiological HCG-mediated fall in TSH.
  • In favour of gestational hyperthyroidism: no history of thyroid disease, no goitre, no TED.
  • If in doubt, TRAb are indicated.
  • Do not do an uptake scan.

Treatment of Graves' Disease in Pregnancy

  • PTU, then consider switch to CBZ after 1st trimester.
  • Avoid block-and-replace except if signs of fetal hyperthyroidism.
  • Monitor TFTs every 2-6 weeks.
  • Goal is reasonable control of thyrotoxicosis with minimum drug use, so T4 at top of, or just above, reference range.

Treatment of Thyrotoxicosis Post-Partum

  • Carbimazole is 1st line. Doses up to 30mg/day are safe.
  • PTU is 2nd line due to concern re hepatotoxicity. Doses up to 300mg/day are safe. No side effects were noted even in women overtreated to the point of hypothyroidism.
  • Note Endo Soc guideline 2007 was explicit about safety of all doses.
  • Goal is reasonable control of thyrotoxicosis with minimum drug use, so T4 at top of, or just above, reference range.

Post-partum Thyroiditis

  • Affects 33-50% of TPO positive women.
  • Requires differentiation from Graves' disease.
  • Measure TRAb, or perform uptake scan (requires temporary cessation of breastfeeding and close contact; current RD&E guidelines are 24 hr for breastfeeding, and close contact restricted to 20 min per hour during the day and 20 min per 4 hours overnight).
  • After thyrotoxic phase, screen for hypothyroid phase every 2/12 until 1 year post-partum.
  • If hypothyroidism is treated, taper T4 dose after 6-12/12.
  • After resolution, if euthyroid, screen annually for hypothyroidism.