How should levothyroxine (Synthroid) be adjusted in a patient with elevated Thyroid-Stimulating Hormone (TSH) and low thyroxine (T4) levels, currently taking 75 micrograms (mcg) of Synthroid?

Adjusting Levothyroxine Dosage for Elevated TSH

The levothyroxine dosage should be increased to bring the serum TSH into the reference range. For a patient with TSH of 7.13 and T4 of 0.91 currently on 75 mcg of Synthroid, increase the dose by 12.5 to 25 mcg (to 87.5-100 mcg) and recheck TSH in 6-8 weeks. 1, 2

Assessment of Current Status

• The patient has subclinical hypothyroidism with an elevated TSH (7.13) and a T4 level (0.91) that is likely in the lower normal range 1
• This represents inadequate replacement therapy on the current dose of 75 mcg of levothyroxine 1
• When subclinical hypothyroidism is noted in levothyroxine-treated patients, the dosage should be adjusted to bring the serum TSH into the reference range 1

Dosage Adjustment Algorithm

Step 1: Determine Need for Dose Adjustment

• TSH >4.5 mIU/L with normal T4 in a patient already on levothyroxine indicates need for dose adjustment 1
• For patients already on levothyroxine with elevated TSH, first assess compliance and appropriateness of dose 1

Step 2: Calculate Appropriate Dose Increase

• Increase levothyroxine dosage by 12.5 to 25 mcg per day 2
• For this patient, increase from 75 mcg to either 87.5 mcg or 100 mcg 2
• The full replacement dose for adults is approximately 1.6 mcg/kg/day, which may help guide target dosing 2, 3

Step 3: Monitoring After Adjustment

• Monitor serum TSH 6-8 weeks after any dosage change 1, 2
• Once a stable dosage is achieved with TSH in normal range, evaluate clinical and biochemical response every 6-12 months 2

Special Considerations

• Age and cardiac status affect dosage adjustments:

  • For elderly patients or those with cardiac disorders, use smaller incremental increases (12.5 mcg) 1
  • For younger patients without cardiac risk, larger incremental increases (25 mcg) may be appropriate 2

• Target TSH range considerations:

  • Whether the target TSH level should be in the lower half of the reference range is controversial 1
  • If the patient continues to have symptoms of hypothyroidism despite TSH in the upper half of normal range, it is reasonable to increase the dose to bring TSH into the lower portion of the reference range 1

Common Pitfalls to Avoid

• Failure to check TSH 6-8 weeks after dose adjustment (checking too early or too late) 2

• Overlooking compliance issues before increasing dose 1

• Ignoring factors that may increase levothyroxine requirements:

  • Pregnancy (requires more frequent monitoring) 1
  • Certain medications that affect absorption 4
  • Gastrointestinal conditions affecting absorption 4

• Adjusting dose based solely on symptoms without laboratory confirmation 5

• Excessive dosing leading to iatrogenic hyperthyroidism, which can increase risk of atrial fibrillation and bone loss 1

By following this structured approach to levothyroxine dose adjustment, you can effectively normalize thyroid function while minimizing risks of under or over-replacement.