What is the appropriate levothyroxine starting dose for a 59‑kg adult woman with a TSH of approximately 99 µIU/mL?

Levothyroxine Dosing for a 59 kg Woman with Markedly Elevated TSH

For a 59 kg woman with a TSH of approximately 99 µIU/mL, start levothyroxine at 94 mcg daily (1.6 mcg/kg/day), which is the full replacement dose recommended for adults under 70 years without cardiac disease. 1

Dosing Calculation and Rationale

• The full replacement dose is 1.6 mcg/kg/day, which for a 59 kg patient equals approximately 94 mcg daily 1, 2
• Round to the nearest available tablet strength: 100 mcg daily is the most practical dose 1
• This TSH level of 99 µIU/mL represents severe overt hypothyroidism requiring prompt full-dose replacement 1, 3

Age and Cardiac Considerations

• If she is under 70 years old AND has no cardiac disease, the full replacement dose (1.6 mcg/kg/day) should be started immediately 1, 2
• If she is over 70 years old OR has any cardiac disease (coronary artery disease, atrial fibrillation, heart failure), start at a lower dose of 25-50 mcg daily and titrate gradually by 12.5-25 mcg every 6-8 weeks 1, 3
• A prospective randomized trial demonstrated that full-dose initiation in cardiac asymptomatic patients is safe and reaches euthyroidism faster than low-dose titration, with no cardiac events documented 4

Critical Safety Assessment Before Starting

• Rule out adrenal insufficiency before initiating levothyroxine, especially if central hypothyroidism is suspected, as thyroid hormone can precipitate adrenal crisis 1
• Measure morning cortisol and ACTH if the patient has hypotension, hyponatremia, or unexplained symptoms suggesting adrenal insufficiency 1
• If adrenal insufficiency is present, start hydrocortisone (20 mg morning, 10 mg afternoon) at least one week before levothyroxine 1

Administration Instructions

• Take levothyroxine 30-60 minutes before breakfast on an empty stomach to maximize absorption 2, 3
• Avoid taking within 4 hours of iron, calcium supplements, or antacids, as these reduce absorption 1, 2
• Changing administration time from morning to evening reduces therapeutic efficacy (TSH increases by 1.47 µIU/mL) 5

Monitoring Protocol

• Recheck TSH and free T4 in 6-8 weeks after starting therapy, as this is the time required to reach steady state 1, 2
• Target TSH range is 0.5-4.5 mIU/L with normal free T4 1
• Adjust dose by 12.5-25 mcg increments every 6-8 weeks until TSH normalizes 1, 2
• Once stable, monitor TSH every 6-12 months or sooner if symptoms change 1

Expected Timeline to Euthyroidism

• The peak therapeutic effect may not be attained for 4-6 weeks after starting treatment 2
• With full-dose initiation, most patients reach euthyroidism within 12-16 weeks 4
• Symptoms and quality of life improve gradually over this period 4

Common Pitfalls to Avoid

• Never start at 25 mcg in a young, cardiac-healthy patient with severe hypothyroidism—this unnecessarily delays symptom relief and is not supported by evidence 4
• Avoid adjusting the dose before 6-8 weeks, as levothyroxine has a long half-life and steady state takes time to achieve 1, 6
• Do not overtitrate—approximately 25% of patients on levothyroxine are unintentionally overtreated with suppressed TSH, increasing risks of atrial fibrillation, osteoporosis, and fractures 1, 6
• Confirm the diagnosis with repeat testing if this is the first elevated TSH, as 30-60% of elevated values normalize spontaneously 1, 7

Special Considerations

• If she is pregnant or planning pregnancy, levothyroxine requirements increase by 25-50% during pregnancy; start treatment immediately and target TSH <2.5 mIU/L in the first trimester 1, 3
• If she has positive anti-TPO antibodies, this confirms autoimmune thyroiditis (Hashimoto's) and predicts a 4.3% annual progression risk to worsening hypothyroidism 1
• Elderly patients with coronary disease are at increased risk of cardiac decompensation even with therapeutic doses, requiring a conservative 25-50 mcg starting dose 1, 6, 3