How should I manage a 29‑week pregnant woman with right thyroid enlargement, subclinical hypothyroidism (TSH 4.73 mIU/L, normal free T4) and a thyroid nodule?

Management of Subclinical Hypothyroidism at 29 Weeks Gestation with Thyroid Nodule

Initiate levothyroxine treatment immediately to achieve a TSH <2.5 mIU/L, as this patient has subclinical hypothyroidism in the third trimester with a TSH of 4.73 mIU/L (above the pregnancy-specific upper limit of ~2.5-3.0 mIU/L). 1

Treatment Rationale and Goals

• Start levothyroxine therapy now because maternal subclinical hypothyroidism (elevated TSH with normal free T4) is associated with pregnancy loss, preterm labor, and lower child cognitive assessment scores, even though treatment trials have not definitively proven benefit 1
• The TSH goal during pregnancy is <2.5 mIU/L, which is more stringent than non-pregnant targets 1
• Women with overt hypothyroidism should be treated to this same TSH goal, and your patient's TSH of 4.73 mIU/L warrants intervention given the potential adverse obstetric and neurodevelopmental outcomes 1

Levothyroxine Dosing Strategy

• Initial dose: Start with 50-75 mcg daily (or calculate 1.2-1.5 mcg/kg of pre-pregnancy weight) 1
• Monitoring frequency: Check TSH and free T4 every 4 weeks until stable, then every 4-6 weeks throughout pregnancy 1
• Dose adjustments: Increase by 12.5-25 mcg increments based on TSH results to maintain TSH <2.5 mIU/L 1

Thyroid Nodule Management

• Defer fine needle aspiration (FNA) unless highly suspicious features are present on ultrasound (marked hypoechogenicity, irregular margins, microcalcifications, taller-than-wide shape, or abnormal lymph nodes) 2
• The thyroid enlargement (thyromegaly) and nodule can be fully evaluated postpartum unless there are concerning ultrasound characteristics or rapid growth 2
• Ultrasound evaluation of the nodule should be performed to assess size and sonographic features, but most nodules can wait until after delivery for definitive workup 2

Critical Monitoring Parameters

• TSH and free T4 every 4 weeks initially, as pregnancy increases levothyroxine requirements by 30-50% in women with pre-existing hypothyroidism 1
• Thyroid peroxidase antibodies (TPOAb) if not already checked, as positive antibodies may indicate autoimmune thyroiditis and higher risk for postpartum thyroid dysfunction 3, 2
• Fetal growth and well-being through routine obstetric monitoring, as untreated hypothyroidism can affect fetal neurodevelopment 1

Common Pitfalls to Avoid

• Do not delay treatment waiting for "more definitive" TSH elevation—the third trimester is critical for fetal brain development and TSH >4.0 mIU/L warrants intervention 1
• Do not use total T4 or T3 levels for monitoring, as pregnancy increases thyroxine-binding globulin 2-3 fold, making total hormone measurements unreliable; always use free T4 4
• Do not perform radioactive iodine scanning for nodule evaluation—this is absolutely contraindicated in pregnancy 5
• Avoid under-dosing levothyroxine out of excessive caution; the risks of inadequate treatment outweigh medication risks 1

Postpartum Planning

• Recheck thyroid function 6 weeks postpartum to detect postpartum thyroiditis or adjust levothyroxine dose, as requirements typically decrease after delivery 6
• Complete nodule evaluation with ultrasound and FNA if indicated based on size (>1 cm) and sonographic features 2
• Continue levothyroxine postpartum and adjust dose based on TSH monitoring, targeting TSH 0.5-2.5 mIU/L 1