In a 31‑year‑old pregnant woman weighing 83 kg with overt hypothyroidism (TSH 27.3 mIU/L, free T4 0.19 ng/dL), can I start levothyroxine 125 µg (Synthroid) now and adjust the dose when she follows up with an endocrinologist?

Can You Start Levothyroxine 125 µg Now and Adjust Later with an Endocrinologist?

Yes, you should start levothyroxine 125 µg immediately—this is overt hypothyroidism in pregnancy, which requires urgent treatment to prevent serious maternal and fetal complications. Waiting for an endocrinologist appointment risks irreversible harm to both mother and baby. 1, 2

Why Immediate Treatment Is Critical

Pregnancy with overt hypothyroidism is a medical emergency. Your patient has:

• TSH 27.3 mIU/L (severely elevated)
• Free T4 0.19 ng/dL (critically low)
• Weight 83 kg
• Age 31 years (reproductive age)

Untreated maternal hypothyroidism during pregnancy causes: 2

• Spontaneous abortion
• Gestational hypertension and pre-eclampsia
• Stillbirth and premature delivery
• Permanent fetal neurocognitive impairment (this damage occurs early and cannot be reversed later)

The 125 µg Starting Dose Is Appropriate

For overt hypothyroidism in a young, otherwise healthy pregnant woman, the full replacement dose should be started immediately. 1

Dose calculation:

• Standard replacement: 1.6 µg/kg/day 1
• For 83 kg: 1.6 × 83 = 132.8 µg/day
• 125 µg is the closest available tablet strength and is appropriate 1

This patient does NOT need a low starting dose because: 1

• She is under 70 years old
• No cardiac disease mentioned
• No multiple comorbidities mentioned
• The guideline explicitly states full replacement dose (1.6 µg/kg/day) is recommended for patients <70 years without cardiac disease 1

Pregnancy-Specific Considerations

Levothyroxine requirements increase 25-50% during pregnancy. 2 Starting at full replacement now positions you to:

• Achieve euthyroidism rapidly (critical for fetal brain development in first trimester)
• Accommodate the pregnancy-related increase in requirements
• Prevent the need for multiple dose escalations during early pregnancy

TSH should be monitored every 4 weeks during pregnancy until stable, then at minimum once per trimester, with a target TSH <2.5 mIU/L in the first trimester. 1

Safety Precautions Before Starting

Before initiating levothyroxine, you MUST rule out concurrent adrenal insufficiency by checking morning cortisol and ACTH, especially if there are any features suggesting central hypothyroidism or autoimmune polyglandular syndrome. 1 Starting thyroid hormone before adequate corticosteroid coverage can precipitate life-threatening adrenal crisis. 1

However, given the TSH of 27.3 mIU/L with low free T4, this is clearly primary hypothyroidism (not central), making adrenal insufficiency less likely—but still check if any clinical suspicion exists (hypotension, hyponatremia, hyperpigmentation). 1

Monitoring and Endocrinologist Follow-Up

Initial monitoring schedule: 1

• Recheck TSH and free T4 in 6-8 weeks after starting treatment
• Adjust dose by 12.5-25 µg increments based on results
• Continue monitoring every 6-8 weeks until TSH normalizes
• During pregnancy, check TSH every 4 weeks until stable

The endocrinologist can and should adjust the dose based on these follow-up labs. This is standard practice—no physician expects the starting dose to be perfect. 1 The goal is to:

1. Start treatment immediately (which you're doing correctly)
2. Monitor response
3. Titrate to target TSH 0.5-4.5 mIU/L (or <2.5 mIU/L in first trimester if pregnant) 1

Common Pitfalls to Avoid

Do NOT delay treatment waiting for specialist consultation. 1, 2 The fetal brain is developing now, and every day of untreated maternal hypothyroidism increases the risk of permanent neurodevelopmental deficits. 2

Do NOT start with a low dose (25-50 µg) in this young, otherwise healthy patient. 1 Low starting doses are reserved for patients >70 years or those with cardiac disease/multiple comorbidities. 1 Starting too low in this pregnant woman will delay achieving euthyroidism and prolong fetal exposure to inadequate thyroid hormone.

Do NOT forget to counsel the patient to take levothyroxine on an empty stomach, 30-60 minutes before breakfast, and at least 4 hours apart from iron, calcium supplements, or antacids (which are commonly taken during pregnancy). 1

Approximately 25% of patients on levothyroxine are unintentionally overtreated, 1 so the endocrinologist's role in fine-tuning the dose based on serial TSH measurements is important—but this should not delay starting appropriate treatment now.

Patient Education

Inform the patient that:

• Levothyroxine is safe during pregnancy and essential for her baby's brain development 2
• She will need lifelong treatment (hypothyroidism is usually permanent) 1
• Dose adjustments during pregnancy are normal and expected 2
• She should report any cardiac symptoms (chest pain, palpitations, shortness of breath) immediately, though these are unlikely at appropriate replacement doses 1

Bottom line: Start 125 µg levothyroxine today, recheck labs in 6-8 weeks, and have the endocrinologist adjust the dose based on those results. This approach prioritizes the immediate health needs of both mother and fetus while ensuring appropriate specialist oversight for ongoing management.