Hypothyroidism in Pregnancy

Hypothyroidism in pregnancy is low thyroid hormone, with raised TSH and low free T4. The commonest causes are autoimmune thyroiditis (Hashimoto disease), previous thyroid surgery or radioiodine, and iodine deficiency. It matters because untreated disease causes miscarriage, preeclampsia, anaemia, abruption, preterm birth, stillbirth and impaired child neurodevelopment, and the levothyroxine dose must rise early in pregnancy.

• Fetal brain development in the first trimester depends on maternal thyroxine.
• Levothyroxine needs rise by 25 to 50 percent in pregnancy.
• Iodine deficiency is a common cause in areas with goitre.

Women have tiredness, cold intolerance, constipation, weight gain, dry skin, hair loss, slow pulse and sometimes goitre, though many symptoms overlap with normal pregnancy.

• Delayed relaxation of ankle reflexes.
• Slow pulse and puffy face in severe disease.
• Differential diagnosis: normal pregnancy tiredness, anaemia, depression, subclinical hypothyroidism, central hypothyroidism.
• Complications: miscarriage, preeclampsia, anaemia, placental abruption, preterm birth, low birth weight, stillbirth, postpartum haemorrhage, impaired child neurodevelopment.

The approach is thyroid function with antibody testing.

• TSH above 4.0 mIU/L with low free T4 confirms overt hypothyroidism; TSH above 10 mIU/L is overt whatever the free T4.
• Thyroid peroxidase antibodies; thyroid ultrasound if goitre or nodule.
• Full blood count, ferritin, lipids if indicated.
• TSH trimester targets: first trimester 0.1 to 2.5 mIU/L, second and third trimesters 0.2 to 3.0 mIU/L.

The principle is full replacement with levothyroxine, with early dose increase and frequent TSH checks.

• Supportive: take levothyroxine on an empty stomach 30 to 60 minutes before breakfast; separate from iron, calcium and antacids by 4 hours; iodised salt in food; stop smoking.
• Pharmacological: new diagnosis: levothyroxine 1.6 to 2.0 micrograms/kg oral once daily (usually 100 to 150 micrograms), starting at full dose; known hypothyroidism: increase the usual dose by 25 to 30 percent (2 extra tablets weekly, 9 doses per week) as soon as pregnancy is confirmed; adjust by 25 to 50 micrograms every 4 weeks to keep TSH at 0.1 to 2.5 mIU/L in the first trimester and below 3.0 mIU/L later; iodine 150 to 250 micrograms oral daily as potassium iodide; ferrous sulphate 200 mg oral 8-hourly if anaemic.
• Procedures: none.
• Special groups: after delivery return to the pre-pregnancy dose and check TSH at 6 weeks; breastfeeding is compatible with levothyroxine; women after thyroidectomy need dose increase of 30 to 50 percent; neonates have heel prick TSH at 3 to 5 days.
• Red flags: TSH above 10 mIU/L, severe tiredness, slow pulse, confusion, hypothermia, fetal growth restriction.
• Follow-up: TSH every 4 weeks until 20 weeks, then at 28 and 34 weeks; growth scans from 28 weeks; thyroid function at 6 weeks postpartum.