Thyroid Disease in Pregnancy

Hypothyroidism

• Several mechanisms can lead to hypothyroidism in pregnancy, including increased urinary iodine excretion, increased thyroxine binding globulin, and increased thyroid hormone degradation by placental type 3 deiodinase.
• Typically, a clinical diagnosis of hypothyroidism in pregnancy can be defined as a TSH greater than 10 mU/L in the first trimester; however, even ranges from 2.5-5 mU/mL have demonstrated adverse maternal and fetal outcomes.
• Hypothyroidism can be difficult to distinguish from symptoms of normal pregnancy, but it is important to treat early, given the association between maternal hypothyroidism and significant adverse gestational and intellectual impairments to the fetus.

Hyperthyroidism and Thyroid Storm

Hyperthyroidism

• Hyperthyroidism is far less common during pregnancy (0.1-0.4% of pregnancies) than hypothyroidism.
  • The most common causes of hyperthyroidism in pregnancy are Graves’ disease (0.1-1% of pregnancies) and human chorionic gonadotropin (hCG)-mediated hyperthyroidism due to gestational transient thyrotoxicosis) (1-3% of pregnancies).²
  • hCG-mediated hyperthyroidism occurs because hCG shares an alpha subunit with TSH, and there is a 38% sequence similarity in their beta subunits.
• Typically, pregnancy is a favorable state for autoimmune processes, and the incidence of Graves hyperthyroidism decreases in the second and third trimesters after a slight increase in the first trimester.

Thyroid Storm

• Thyroid storm is a serious and potentially life-threatening thyrotoxicosis condition where excessive circulating thyroid hormone leads to multiorgan dysfunction and systemic decompensation.
• Patients will exhibit severe symptoms of:
  • Hyperpyrexia (more than 103°F)
  • Tachycardia (usually more than 140bpm) and tachyarrhythmias
  • Signs of congestive heart failure (shortness of breath, edema, etc.)
    • Pregnant patients are at higher risk of developing cardiomyopathy, heart failure, and tachyarrhythmias.
  • Nausea, vomiting, diarrhea
  • Neuropsychiatric disturbances: confusion, restlessness, altered mental status, tremors, seizures.
• Management of thyroid storm includes a multidisciplinary team of intensivists, endocrinologists, and maternal-fetal medicine specialists.

Intraoperative Management of Thyroid Storm

• Clinicians should secure vascular access and provide oxygen therapy.
• A low threshold for invasive blood pressure monitoring and endotracheal intubation should be maintained.
• Clinicians should ensure adequate crystalloid administration (to replace gastrointestinal and insensible losses), aggressive management and treatment of cardiac complications (beta blockers for tachycardia/tachyarrhythmias and hypertension), thiamine administration (with guidance from endocrine teams), and euthermia (with a cooling blanket).
• Acetaminophen should be administered for antipyogenic effects.
  • Aspirin should be avoided as it can theoretically displace thyroid hormone from the serum binding site, increasing free thyroid levels.

Thyroid Medications

Hypothyroid Medications

• Levothyroxine is the mainstay of thyroid hormone replacement.
  • Pregnancy dosing typically increases by 30%-50% compared to the prepregnancy dose, and thyroid function tests should be performed every 4-6 weeks to ensure adequate response to TSH levels.⁵

Hyperthyroid Medications