A patient with a TSH of 13.6 mIU/L who is currently taking Synthroid (levothyroxine) 112 µg daily, what dose adjustment should be made?

Increase Synthroid to 125 mcg daily

With a TSH of 13.6 mIU/L on 112 mcg daily, increase the levothyroxine dose by 12.5-25 mcg to 125 mcg daily, then recheck TSH in 6-8 weeks.

Rationale for Dose Adjustment

The current TSH of 13.6 mIU/L indicates clear undertreatment—this patient remains hypothyroid despite therapy. According to AACE/ATA guidelines, increment changes of 12.5-25 mcg/day are the standard approach for dose adjustments 1. The FDA labeling for Synthroid confirms that levothyroxine doses are generally adjusted in 12.5-25 mcg increments until the patient is clinically euthyroid and serum TSH has normalized 2.

Specific Dosing Algorithm

• Current dose: 112 mcg daily
• Recommended increase: 12.5-25 mcg
• New dose options: 125 mcg (13 mcg increase) or 137 mcg (25 mcg increase)
• Preferred choice: 125 mcg daily represents a conservative 12.5 mcg increase that minimizes risk of overcorrection

The 125 mcg dose is preferable because:

• It provides adequate upward titration without overshooting
• Standard tablet strengths make this practical
• Allows room for further adjustment if needed

Critical Timing Considerations

Recheck TSH in 6-8 weeks after the dose change 1, 2. This timing is essential because:

• The long half-life of levothyroxine means peak therapeutic effect takes 4-6 weeks to manifest 2
• TSH levels require 8 weeks or longer to plateau when making smaller dose adjustments 1
• Earlier testing will not reflect steady-state levels and may lead to inappropriate further adjustments

Important Caveats Before Adjusting

Before increasing the dose, verify:

1. Medication compliance - Non-compliance is the most common cause of persistently elevated TSH 1. Patients on doses >200 mcg/day with elevated TSH are frequently non-compliant 1

2. Proper administration - Levothyroxine should be taken:

   • 30-60 minutes before breakfast with water 1
   • At least 4 hours apart from interfering medications 2

3. Absorption interference - Check for medications/supplements that reduce absorption 1:

   • Iron supplements
   • Calcium supplements
   • Antacids
   • Proton pump inhibitors
   • Bile acid sequestrants

4. Patient age and cardiac status - If this patient is >50-60 years old or has coronary heart disease, smaller increments (12.5 mcg) are safer 1, 2

Special Populations Requiring Modified Approach

If the patient is elderly (>60 years) or has known coronary disease, use the 12.5 mcg increment (to 125 mcg) rather than 25 mcg, and monitor for anginal symptoms 1. The FDA specifically warns that patients with coronary artery disease should be monitored closely as levothyroxine may precipitate cardiac arrhythmias or coronary insufficiency 2.

When to Suspect Non-Standard Situations

If TSH remains elevated after appropriate dose adjustment and adequate time:

• Consider malabsorption issues
• Evaluate for drug interactions
• Assess for decreased potency of the levothyroxine preparation
• Rule out adrenal insufficiency (rare but can cause elevated TSH with normal T4) 1

The goal is to normalize TSH to the reference range (typically 0.4-4.0 mIU/L), which correlates with symptom resolution and optimal metabolic function while avoiding the cardiovascular and skeletal risks of overtreatment 1, 2.