Can pregnancy trigger hyperthyroidism?

Can Pregnancy Trigger Hyperthyroidism?

Yes, pregnancy can trigger hyperthyroidism through two distinct mechanisms: gestational transient thyrotoxicosis (caused by high hCG levels) and exacerbation or new onset of Graves' disease due to immune system changes.

Pregnancy-Specific Forms of Hyperthyroidism

Gestational Transient Thyrotoxicosis

• Hyperemesis gravidarum is associated with biochemical hyperthyroidism, including undetectable TSH levels and elevated free thyroxine index (FTI), due to high hCG levels during the first trimester 1
• This condition typically occurs in the first trimester when hCG levels peak and has structural similarity to TSH, directly stimulating the thyroid gland 2
• This form rarely requires treatment as it is self-limiting and resolves as hCG levels decline 1

Graves' Disease in Pregnancy

• Graves' disease is responsible for 95 percent of hyperthyroidism cases in pregnancy and represents the most common pathological cause 3
• The physiological changes in the maternal immune system during pregnancy may influence the development of autoimmune diseases like Graves' disease 4
• Pre-existing Graves' disease may worsen or improve during pregnancy due to immune system fluctuations 4

Diagnostic Approach

Laboratory Testing

• TSH testing is the initial test for screening, with both TSH and free T4 (FT4) or free thyroxine index (FTI) performed in pregnant women with suspected hyperthyroidism 1
• In hyperthyroidism, TSH is suppressed with elevated FT4 or FTI 1
• Diagnosis is based on elevated free thyroxine (FT4) level or free thyroxine index (FTI) with suppression of TSH in the absence of thyroid mass or nodular goiter 3

Clinical Features to Identify

• Tremors, nervousness, insomnia, excessive sweating, heat intolerance, tachycardia, hypertension, and goiter 3
• Distinctive ophthalmic signs include eyelid lag or retraction; dermal signs include localized and pretibial myxedema (specific to Graves' disease) 3
• Bodyweight loss, eye signs, and a bruit over the thyroid gland warrant thyroid investigation 5

Critical Pitfall to Avoid

• Failure to use trimester-specific TSH reference ranges can lead to misinterpretation of thyroid function tests and underdiagnosis 1
• Total T4 and T3 levels may be raised in euthyroid pregnancies due to increased thyroxine binding globulin (TBG), so free hormone levels must be measured 5

Maternal and Fetal Risks of Untreated Disease

Maternal Complications

• Unless hyperthyroidism is treated adequately, pregnant women are at increased risk for severe preeclampsia, preterm delivery, heart failure, and possibly miscarriage 3
• Thyroid storm is a rare medical emergency affecting a small percentage of pregnant women with hyperthyroidism 3

Fetal and Neonatal Risks

• Low birth weight in neonates can occur with inadequately treated maternal hyperthyroidism 3
• Fetal thyrotoxicosis needs to be considered in women who have a history of Graves' disease, as antibodies cross the placenta 3
• The possibility of neonatal immune-mediated hypothyroidism or hyperthyroidism is an additional concern due to transplacental antibody passage 3
• Fetal hyperthyroidism can be life-threatening and requires treatment of the fetus with antithyroid drugs via the mother 2

Post-Partum Considerations

• Post-partum thyroiditis occurs in 5-10% of women, and many of those affected ultimately develop permanent hypothyroidism 2
• This represents another pregnancy-related trigger for thyroid dysfunction that manifests after delivery 2