I am taking levothyroxine (T4) 37.5 µg; should I continue this dose, obtain thyroid function tests, or add liothyronine (T3)?

Obtain Thyroid Function Tests Now

You should obtain thyroid function tests (TSH and free T4) within 4-8 weeks if you recently started or adjusted your levothyroxine dose, or if you haven't had testing in the past 6-12 months. 1 Do not add liothyronine (T3) at this time—levothyroxine monotherapy remains the standard of care and should be optimized first.

Monitoring Timeline and Dose Optimization

Your current dose of 37.5 mcg is relatively low, suggesting either:

• You're in the early titration phase of treatment
• You have mild hypothyroidism
• You're elderly or have cardiac risk factors

Key monitoring intervals:

• 4-8 weeks after starting therapy or any dose change: TSH testing is mandatory to guide adjustments 1
• Once stable: TSH testing every 6-12 months 1
• Smaller doses (like 37.5 mcg) may require 8 weeks or longer before TSH levels plateau, so don't rush adjustments 1

Target TSH and Dose Adjustments

Your therapeutic goal is achieving a TSH in the normal reference range (typically 0.4-4.12 mIU/L). 1 The most reliable endpoint for primary hypothyroidism treatment is serum TSH—not how you feel, not free T4 alone. 1

When adjusting doses:

• Make incremental changes of 12.5-25 mcg daily 1
• Wait 4-8 weeks between adjustments 1
• The full therapeutic effect of any dose may take 4-6 weeks to manifest 2

Common pitfall: Patients often require doses >200 mcg/day with persistently elevated TSH—this usually indicates non-compliance (most common) or absorption problems, not inadequate dosing. 1

Why Not Add Liothyronine (T3)?

Levothyroxine monotherapy should remain your treatment. 3 Multiple randomized trials over 20+ years have failed to show consistent benefit of adding T3 to T4 therapy. 4, 5, 3

T3 combination therapy is only considered after:

1. Confirmed overt hypothyroidism (not subclinical)
2. Optimized levothyroxine with TSH 0.3-2.0 mIU/L for 3-6 months
3. Persistent symptoms despite optimal dosing
4. Exclusion of other comorbidities
5. Shared decision-making discussion 4

Even then, evidence shows no consistent improvement in quality of life, though some patients express preference for combination therapy. 6, 3 The safety profile appears acceptable, but efficacy remains unproven. 7

Critical Factors Affecting Your Dose

Check if any of these apply to you:

• Medications that interfere with absorption: Take levothyroxine 30-60 minutes before breakfast on an empty stomach, away from calcium, iron, proton pump inhibitors, bile acid sequestrants 1
• Age: Elderly patients require 20-25% less per kilogram than younger patients 1
• Weight changes: Significant weight loss decreases requirements 1
• Cardiac disease: If you have coronary disease, slower titration is essential to avoid angina 1

Avoiding Overtreatment

20% of patients on thyroid hormone are overtreated. 1 The consequences include:

• Cardiovascular complications (especially atrial fibrillation in elderly) 1
• Accelerated bone loss (especially in postmenopausal women) 1
• Possible mood disturbances 1

Your action plan:

1. Schedule TSH and free T4 testing if not done recently
2. Continue current 37.5 mcg dose until results available
3. Adjust dose in 12.5-25 mcg increments based on TSH
4. Recheck TSH 4-8 weeks after any adjustment
5. Once stable, monitor every 6-12 months