Thyroid Disease in Pregnancy

There is a spectrum/classification of disease as follows. These disorders are managed differently than in non-pregnancy individuals.

1. Subclinical hypothyroidism
2. Pre-existing hypothyroidism
3. Grave's Disease
4. Gestational Hyperthyroidism

Subclinical Hypothyroidism

Informed by the 2017 American Thyroid Association guidelines. Managed differently than patients with pre-existing hypothyroidism (see below). Management will depend on the TSH level +/- presence of TPOAb (which is a modifying factor which if present warrants treatment).

Arguments can be made for the use of different TSH cutoffs--as opposed to an UL of 4.0, take the population ULN and subtract 0.5. However, the guidelines will still say 4 so this will be the standard and exam answer.

Pre-Existing Hypothyroidism

• pregnancy increases one's requirement for active T4 by ~20-30%. Therefore, for all pregnant women, increase their LT4 as soon as diagnosed with pregnancy.
  • Practically this can be done by taking an extra LT4 pill on Sat/Sun (+28% weekly dose)
• On delivery, resume the pre-pregnancy dose
• Target TSH ≤ 2.5 throughout the pregnancy

Graves' Disease in Pregnancy

• Try to avoid antithyroidal medications as they are teratogenic
  • First trimester - PTU
  • After first trimester - Methimazole (or discontinue all ATDs)
  • use the lowest possible dose (aim for a T4 in the high-normal range)
• Other considerations
  • TR-Ab can cross the placenta -- check the Ab titre in the second trimester. If very high (>3x ULN), then need to increase fetal monitoring for fetal Graves'
  • Long-term treatment with beta-blockade has been associated with IUGR, fetal bradycardia, and neonatal hypoglycemia
  • Post-partum need to monitor for thyroiditis exacerbations

Gestational Transient Thyrotoxicosis

• Pathophysiology: in pregnancy, TBG and tF4 increase by 7 wk GA and peak at 16 wk GA. hCG stimulates the TSH receptor causing downstream hyperthyroidism
  • This hCG-mediated effect is more pronounced in HG, molar pregnancy, multiple gestation pregnancy, choriocarcinoma
• Rule out other issues (Graves, molar pregnancy)
  • ?molar pregnancy: Pelvic US
  • ?Graves eye disease, thyroid bruit, goitre, TRAb+, nodules
• Management
  • generally self-limited and improves by 14-18 weeks GA
  • Treat hyperemesis if present
  • Use beta-blockade sparingly for symptoms

References

1. 2021 IMR Slides
2. 2017 ATA Guidelines