What is the appropriate starting dose of levothyroxine (T4) for a postpartum patient with hypothyroidism, weighing 106 kg, with a Thyroid-Stimulating Hormone (TSH) level of 7.5 and a free thyroxine (free T4) level of 0.85, who is treatment-naive?

Starting Levothyroxine Dose for Postpartum Hypothyroidism

For this 106 kg postpartum patient with overt hypothyroidism (TSH 7.5, free T4 0.85), start levothyroxine at approximately 170 mcg daily (1.6 mcg/kg/day), which is the full replacement dose recommended for adults under 70 years without cardiac disease. 1, 2

Rationale for Full Replacement Dosing

• Your patient has overt hypothyroidism with both elevated TSH (7.5 mIU/L) and low free T4 (0.85), not subclinical hypothyroidism 1
• The guideline-recommended starting dose is 1.6 mcg/kg/day for patients under 70 years without cardiac disease or multiple comorbidities 1, 2
• At 106 kg body weight: 1.6 mcg/kg × 106 kg = approximately 170 mcg daily 1, 2
• Round to the nearest available tablet strength (typically 175 mcg or 150 mcg + 25 mcg) 2

Special Postpartum Considerations

• Postpartum patients can tolerate full replacement dosing without the gradual titration needed in elderly or cardiac patients 1
• The postpartum period does not require dose reduction compared to pregnancy; in fact, many women needed higher doses during pregnancy and should return to pre-pregnancy levels immediately after delivery 2
• Since this patient was never on levothyroxine before, she doesn't have a pre-pregnancy baseline, so standard adult dosing applies 1, 2

Monitoring Protocol

• Recheck TSH and free T4 in 6-8 weeks after starting therapy to assess response 1, 3
• Target TSH within the reference range (0.5-4.5 mIU/L) with normalized free T4 1, 3
• Adjust dose by 12.5-25 mcg increments if needed based on follow-up labs 1, 3
• Once stabilized, monitor every 6-12 months or if symptoms change 1

Critical Pitfalls to Avoid

• Do not start at 25-50 mcg daily unless the patient is over 70 years old or has cardiac disease—this low starting dose is inappropriate for younger, otherwise healthy adults and will prolong the time to achieving euthyroidism 1, 4
• Do not underdose based on the TSH being "only" 7.5—the low free T4 confirms this is overt hypothyroidism requiring full replacement 1
• Ensure the patient takes levothyroxine on an empty stomach (30 minutes before breakfast) to optimize absorption 5
• Avoid iron, calcium supplements, or proton pump inhibitors within 4 hours of levothyroxine as these reduce absorption 1
• If she is breastfeeding and plans future pregnancy, counsel that levothyroxine requirements typically increase by 30% during pregnancy and she should contact you immediately when pregnant 2, 6

Why Not Lower Starting Doses?

• Lower starting doses (25-50 mcg) are reserved for patients over 70 years or with known cardiac disease/multiple comorbidities 1, 4
• Starting too low in a young, otherwise healthy patient unnecessarily prolongs hypothyroid symptoms and delays achieving therapeutic levels 1
• The full replacement dose of 1.6 mcg/kg/day is safe and appropriate for this patient population 1, 2, 7