Subclinical Hypothyroidism in Pregnancy

Subclinical hypothyroidism in pregnancy is a raised TSH with normal free T4. It is usually autoimmune. It matters because TSH above 4 mIU/L, and especially with thyroid antibodies, raises the risk of miscarriage, preterm birth and preeclampsia, and treatment lowers some risks.

• TSH is normally lower in the first trimester than outside pregnancy.
• Thyroid peroxidase antibodies increase the risk.
• Isolated low free T4 with normal TSH is not treated.

Most women have no symptoms. Some have tiredness, constipation or mild goitre.

• Often found on routine thyroid testing or in miscarriage clinics.
• Examination is usually normal.
• Differential diagnosis: normal pregnancy variation, overt hypothyroidism, nonthyroidal illness, assay interference, recovery phase of thyroiditis.
• Complications: progression to overt hypothyroidism, miscarriage, preterm birth, preeclampsia, placental abruption, impaired child neurodevelopment.

The approach is TSH, free T4 and antibodies.

• TSH above 2.5 mIU/L in the first trimester, or above 3.0 mIU/L later, with normal free T4 confirms the diagnosis.
• Thyroid peroxidase antibodies to guide treatment.
• Repeat TSH and free T4 in 4 weeks to confirm.
• Ferritin and full blood count.

The principle is levothyroxine when TSH is above 4 mIU/L or when antibodies are positive with TSH above 2.5 mIU/L.

• Supportive: iodised salt; take levothyroxine on an empty stomach 30 to 60 minutes before breakfast; separate from iron and calcium by 4 hours.
• Pharmacological: for TSH 2.5 to 4.0 mIU/L with positive antibodies, or TSH above 4.0 mIU/L, start levothyroxine 50 micrograms oral once daily and increase by 25 micrograms every 4 weeks to keep TSH below 2.5 mIU/L (first trimester) or below 3.0 mIU/L (later); for TSH above 10 mIU/L use levothyroxine 1.6 micrograms/kg oral daily; iodine 150 to 250 micrograms oral daily; do not treat TSH below 2.5 mIU/L with negative antibodies.
• Procedures: none.
• Special groups: after delivery stop levothyroxine if the starting dose was 50 micrograms or below and recheck TSH at 6 weeks; women with positive antibodies are tested yearly because progression occurs.
• Red flags: TSH rising above 10 mIU/L, goitre growth, severe tiredness, fetal growth restriction.
• Follow-up: TSH every 4 weeks until 20 weeks, then at 28 and 34 weeks; thyroid function at 6 weeks postpartum and yearly.