Levothyroxine Should NOT Be Decreased When TSH Is Normal
If TSH is within the normal reference range (0.45-4.5 mIU/L) in a patient taking levothyroxine for hypothyroidism, the dose should be maintained, not decreased. 1
When Dose Reduction Is Actually Indicated
Levothyroxine dose reduction is only appropriate when TSH falls below the normal range, indicating overtreatment:
For TSH 0.1-0.45 mIU/L: Reduce dose by 12.5-25 mcg, particularly if TSH is in the lower part of this range or in patients with atrial fibrillation, cardiac disease, or elderly patients with risk factors for cardiac complications 1
For TSH <0.1 mIU/L: Decrease dose by 25-50 mcg immediately, as severe TSH suppression significantly increases risks for atrial fibrillation, osteoporosis, fractures, and cardiovascular mortality 1
Target TSH Range for Primary Hypothyroidism
The goal of levothyroxine therapy is to maintain TSH within the reference range of 0.5-4.5 mIU/L with normal free T4 levels 1, 2. A normal TSH indicates the current dose is appropriate and achieving the therapeutic goal 1.
Critical Distinction: Hypothyroidism vs. Thyroid Cancer
For patients with thyroid cancer, TSH targets differ based on risk stratification and intentional TSH suppression may be required 1:
- Low-risk patients with excellent response: TSH 0.5-2 mIU/L 1
- Intermediate-to-high risk patients: TSH 0.1-0.5 mIU/L 1
- Structural incomplete response: TSH <0.1 mIU/L 1
However, for primary hypothyroidism without thyroid cancer, maintaining TSH in the normal range is the goal, and dose reduction when TSH is normal would risk undertreatment 1, 2.
Risks of Inappropriate Dose Reduction
Reducing levothyroxine when TSH is already normal will cause:
- Persistent hypothyroid symptoms including fatigue, weight gain, cold intolerance, and cognitive impairment 1
- Adverse cardiovascular effects including delayed cardiac relaxation and abnormal cardiac output 1
- Adverse lipid profiles with elevated LDL cholesterol 1
- Decreased quality of life 1
- Risk of progression to overt hypothyroidism 1
Monitoring Recommendations
Once TSH is stable within the normal range:
- Recheck TSH every 6-12 months 1
- Recheck sooner if symptoms change or new medications are started that may affect thyroid function 1
- Both TSH and free T4 should be measured if TSH becomes abnormal to distinguish between undertreatment and overtreatment 1
Common Pitfall to Avoid
Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for atrial fibrillation, osteoporosis, fractures, and cardiac complications 1. However, the opposite error—reducing dose when TSH is appropriately controlled—leads to undertreatment and return of hypothyroid symptoms and complications 1, 2.
The key principle: Normal TSH = appropriate dose. Only reduce levothyroxine when TSH is suppressed below normal, indicating overtreatment. 1, 2