Thyrotoxicosis in Pregnancy

Thyrotoxicosis in pregnancy is excess thyroid hormone, with suppressed TSH and raised free T4 or free T3. Graves disease is the commonest cause; others are gestational transient thyrotoxicosis from hCG, toxic nodule, subacute thyroiditis and molar pregnancy. It matters because uncontrolled disease causes miscarriage, preeclampsia, preterm birth, growth restriction and thyroid storm.

• Gestational transient thyrotoxicosis settles by 14 to 18 weeks and needs no antithyroid drug.
• Antithyroid drugs carry fetal risks that guide the choice and timing.
• Antibody tests separate Graves disease from transient disease.

Women have weight loss, tremor, palpitations, heat intolerance, sweating, anxiety and diarrhoea, with tachycardia above 100 per minute and sometimes a goitre and eye signs. Hyperemesis suggests transient thyrotoxicosis.

• Weight loss despite appetite.
• Resting pulse above 100 per minute that does not fall with rest.
• Differential diagnosis: normal pregnancy symptoms, gestational transient thyrotoxicosis, Graves disease, toxic nodule, thyroiditis, anxiety, molar pregnancy.
• Complications: thyroid storm, heart failure, atrial fibrillation, miscarriage, preeclampsia, preterm birth, fetal growth restriction, fetal or neonatal thyrotoxicosis.

The approach is thyroid function with antibodies, with trimester adjusted interpretation.

• TSH below 0.1 mIU/L with raised free T4 (and free T3) confirms thyrotoxicosis.
• TSH receptor antibodies and thyroid peroxidase antibodies; positive receptor antibodies indicate Graves disease.
• Full blood count, liver enzymes (baseline before antithyroid drugs), ECG, ultrasound of pregnancy and thyroid.
• Ultrasound for fetal growth and heart rate from 20 weeks if receptor antibodies are above 3 times normal.

The principle is to treat persistent true hyperthyroidism with the lowest effective dose of an antithyroid drug and no treatment for transient disease.

• Supportive: rest; avoid excess iodine; admit for storm or heart failure; advise to report sore throat or fever at once.
• Pharmacological: propylthiouracil 100 mg oral 8-hourly in the first trimester (up to 16 weeks), adjusted to keep free T4 at the upper limit of normal, then change to carbimazole 10 to 20 mg oral once daily after 16 weeks, adjusted by free T4 every 2 to 4 weeks; propranolol 10 to 40 mg oral 8-hourly for symptoms for a short period only; for thyroid storm give propylthiouracil 600 to 1000 mg oral or by nasogastric tube then 250 mg 4-hourly, propranolol 1 to 2 mg IV slowly or 60 mg oral 4-hourly, hydrocortisone 100 mg IV 8-hourly, potassium iodide 5 drops oral 8-hourly started 1 hour after the first propylthiouracil dose, paracetamol 1 g oral 6-hourly and cooling; do not give radioactive iodine.
• Procedures: subtotal thyroidectomy in the second trimester for severe drug reaction or non response.
• Special groups: breastfeeding is compatible with carbimazole up to 20 mg daily or propylthiouracil up to 300 mg daily taken after feeds in divided doses; neonates of women with positive receptor antibodies have thyroid tests at days 3 to 5 and 10 to 14.
• Red flags: heart rate above 120 per minute, temperature above 38 degrees Celsius, agitation, vomiting, sore throat with fever, jaundice.
• Follow-up: free T4 and TSH every 2 to 4 weeks; white cell count if fever or sore throat; growth scans every 4 weeks from 28 weeks; thyroid tests at 6 weeks postpartum.