What is the appropriate starting dose of levothyroxine (Synthroid) for a 31‑year‑old pregnant woman with newly diagnosed Hashimoto’s thyroiditis, weight 83 kg, TSH 27.3 mIU/L and free T4 0.19 ng/dL?

Starting Levothyroxine Dose for Pregnant Woman with Severe Overt Hypothyroidism

Start levothyroxine immediately at approximately 133 µg daily (1.6 mcg/kg × 83 kg) without waiting for confirmatory testing, because this 31-year-old pregnant woman has severe overt hypothyroidism (TSH 27.3 mIU/L, free T4 0.19 ng/dL) that poses urgent risks to both maternal health and fetal neurodevelopment.

Immediate Dosing Strategy

For new-onset overt hypothyroidism in pregnancy with TSH ≥10 mIU/L, the FDA-approved starting dose is 1.6 mcg/kg/day 1. At 83 kg body weight, this calculates to approximately 133 µg daily 1. Round to the nearest available tablet strength: start 125 µg daily or 137 µg daily 1.

• The full replacement dose should be initiated immediately rather than titrating gradually, because pregnant women require rapid restoration to euthyroidism to prevent fetal harm 2, 3
• Do not use a lower starting dose despite pregnancy, because the urgency of correcting severe maternal hypothyroidism outweighs concerns about gradual titration 2, 3
• This patient's TSH of 27.3 mIU/L with low free T4 represents severe overt hypothyroidism requiring aggressive treatment 4, 1

Critical Safety Precaution Before Starting Levothyroxine

Before administering the first levothyroxine dose, obtain morning (8 AM) serum cortisol and ACTH levels to exclude adrenal insufficiency 4.

• Starting thyroid hormone before adequate glucocorticoid coverage can precipitate life-threatening adrenal crisis 4, 2
• If morning cortisol is low or clinical features suggest adrenal insufficiency (hypotension, hyponatremia), initiate hydrocortisone 20 mg morning and 10 mg afternoon for at least one week before levothyroxine 4
• In Hashimoto's thyroiditis, screen for concurrent autoimmune adrenal insufficiency (Addison's disease) 4

Pregnancy-Specific Considerations

The levothyroxine requirement will increase further as pregnancy progresses 2, 3:

• Pregnant women with pre-existing hypothyroidism typically require a 25–50% dose increase above pre-pregnancy levels 2, 3
• Since this is new-onset hypothyroidism in pregnancy, the initial dose of 1.6 mcg/kg/day already accounts for pregnancy-related increased requirements 1
• Monitor TSH and free T4 every 4 weeks until stable, then at minimum once per trimester 2, 1
• Target TSH <2.5 mIU/L in the first trimester, then within trimester-specific reference ranges 2, 5

Monitoring Protocol

Recheck TSH and free T4 in 4 weeks (not the standard 6–8 weeks for non-pregnant patients) 2, 1:

• Pregnancy requires more frequent monitoring due to rapidly changing thyroid hormone requirements 2, 3
• Adjust levothyroxine dose by 12.5–25 µg increments based on TSH results 2, 1
• Continue monitoring every 4 weeks until TSH is <2.5 mIU/L and stable 2, 1
• After delivery, immediately reduce levothyroxine to pre-pregnancy dose and recheck TSH 4–8 weeks postpartum 1

Rationale for Urgent Treatment

Untreated or inadequately treated maternal hypothyroidism causes severe fetal and maternal complications 2, 3:

• Fetal neurodevelopmental impairment: First-trimester hypothyroidism specifically causes permanent cognitive deficits in offspring 2, 3
• Maternal complications: Preeclampsia, placental abruption, gestational hypertension 2
• Fetal complications: Low birth weight, stillbirth, premature delivery, fetal death 2, 3
• The fetus relies entirely on maternal thyroid hormone during the first and second trimesters for normal brain development 2

Hashimoto's Thyroiditis–Specific Factors

This patient's Hashimoto's thyroiditis diagnosis has important implications 6, 7, 8:

• The presence of anti-TPO antibodies (confirming Hashimoto's) is associated with 2–4-fold increased risk of recurrent miscarriage and preterm birth 6
• Approximately 50% of pregnant women with Hashimoto's thyroiditis require levothyroxine doses at least 20% above baseline even postpartum, indicating progressive autoimmune destruction 8
• Residual thyroid function varies widely in Hashimoto's; this patient's severe TSH elevation (27.3 mIU/L) suggests minimal remaining thyroid reserve 7, 8

Common Pitfalls to Avoid

Do not wait for TSH results before starting levothyroxine in a pregnant woman with confirmed severe hypothyroidism—fetal harm occurs before maternal symptoms appear 2.

Do not start with a low dose (25–50 µg) and titrate gradually as you would in elderly or cardiac patients—pregnancy requires immediate full replacement 4, 1.

Do not target TSH >2.5 mIU/L in the first trimester—even subclinical hypothyroidism (TSH 4.5–10 mIU/L with normal free T4) is associated with adverse pregnancy outcomes 2, 5.

Do not use combination T4/T3 therapy during pregnancy—only levothyroxine monotherapy is indicated, as T3 does not adequately cross the fetal blood-brain barrier 6.

Do not assume the dose can be reduced after the first trimester—levothyroxine requirements typically plateau by week 16 but remain elevated until delivery 3.