From the FDA Drug Label
Titrate the dosage (every 2 weeks) as needed based on serum TSH or free-T4 until the patient is euthyroid Monitor TSH and total or free-T4 in pediatric patients as follows: 2 and 4 weeks after the initiation of treatment, 2 weeks after any change in dosage, and then every 3 to 12 months thereafter following dosage stabilization until growth is completed The general aim of therapy is to normalize the serum TSH level Failure of the serum T4 to increase into the upper half of the normal range within 2 weeks of initiation of levothyroxine sodium therapy and/or of the serum TSH to decrease below 20 IU per litre within 4 weeks may indicate the patient is not receiving adequate therapy
To adjust Synthroid (levothyroxine) if TSH is low and T4 is high, the dosage should be titrated based on serum TSH or free-T4 levels. The goal is to normalize the serum TSH level.
- Decrease the dosage of levothyroxine to avoid over-suppression of TSH and potential adverse effects.
- Monitor TSH and total or free-T4 levels regularly, ideally every 2 weeks after any change in dosage, to assess the adequacy of therapy and make further adjustments as needed 1.
From the Research
If TSH is low and T4 is high, you should reduce the Synthroid (levothyroxine) dose to prevent overtreatment and potential complications associated with hyperthyroidism. Contact your healthcare provider before making any changes, as they will typically decrease your current dose by 12.5-25 mcg per day. For example, if you're taking 100 mcg daily, they might reduce it to 75-87.5 mcg daily. After the dose adjustment, you'll need follow-up thyroid function tests in 6-8 weeks to ensure your levels are normalizing. This adjustment is necessary because your current symptoms indicate hyperthyroidism (overtreatment), where excessive thyroid hormone is suppressing TSH production by the pituitary gland while elevating T4 levels. Signs of hyperthyroidism may include anxiety, tremors, heat intolerance, weight loss, or rapid heartbeat. Never stop Synthroid abruptly, as this can cause hypothyroid symptoms. Dose adjustments for elderly patients or those with heart conditions may need to be more gradual to prevent complications.
Some studies have shown that combined T4 and T3 therapy may be beneficial for some patients, but the evidence is not strong enough to support this as a standard treatment approach 2. In fact, a recent systematic review and meta-analysis found that combined therapy and desiccated thyroid extract (DTE) lead to higher T3 and lower T4 levels compared to T4 monotherapy, but did not result in significant effects on heart rate, lipid profile, or quality of life 2.
It's also important to note that the relationship between serum free T4 and free T3 is complex, and normalization of serum TSH with L-T4 replacement therapy alone may not result in a normal serum L-T3 concentration 3. However, the most recent and highest quality study on this topic is the 2024 systematic review and meta-analysis, which found that combined therapy and DTE may be beneficial for some patients, but more research is needed to determine the optimal treatment approach 2.
Key points to consider when adjusting Synthroid dose:
- Reduce the dose by 12.5-25 mcg per day if TSH is low and T4 is high
- Monitor thyroid function tests 6-8 weeks after dose adjustment
- Be cautious when adjusting doses in elderly patients or those with heart conditions
- Consider the potential benefits and risks of combined T4 and T3 therapy or DTE, but prioritize the most recent and highest quality evidence 2.