Why is mild hyperthyroidism common in hyperemesis, and how is it managed?

hCG shares structural similarity with TSH and has weak thyroid-stimulating activity; in high-hCG states (hyperemesis, multiple/molar pregnancy), this causes a transient gestational (biochemical) hyperthyroidism.

It is self-limiting and resolves as hCG falls (by ~20 weeks) without antithyroid drugs - it is distinguished from true Graves' disease (no goitre, no eye signs, no thyroid antibodies).

Clinical relevance. Recognising gestational transient hyperthyroidism avoids unnecessary (and harmful) antithyroid treatment in hyperemesis.

More Detail

Gestational transient hyperthyroidism in hyperemesis

A common biochemical finding in HG that does not need antithyroid treatment.

Mechanism

• hCG is structurally similar to TSH and has weak thyroid-stimulating activity.
• In high-hCG states (hyperemesis, multiple and molar pregnancy), hCG stimulates the thyroid, causing mild transient hyperthyroidism.
• ~30-73% of women with hyperemesis have abnormal thyroid function tests early in pregnancy (suppressed TSH, mildly raised free T4).

Management

• It is transient and resolves spontaneously as hCG declines (by ~20 weeks).
• No antithyroid treatment is needed (treating could cause fetal hypothyroidism).

Distinguishing from Graves' disease

• Gestational transient hyperthyroidism has no goitre, no eye signs (ophthalmopathy), and negative thyroid antibodies (no TSH-receptor antibodies), unlike Graves'.