From the FDA Drug Label
In adult patients with primary hypothyroidism, monitor serum TSH levels after an interval of 6 to 8 weeks after any change in dosage. 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
The patient's TSH and T4 levels can be rechecked 6 to 8 weeks after any change in dosage, but if there's no improvement in T4 levels within 2 weeks or TSH levels within 4 weeks, the patient may not be receiving adequate therapy 1.
From the Research
If both TSH and T4 are low while taking Synthroid (levothyroxine), you should recheck labs in about 4-6 weeks after any medication adjustment. This situation suggests you may be overmedicated, as low TSH typically indicates excessive thyroid hormone, but the low T4 is inconsistent with this pattern. Your doctor might need to adjust your Synthroid dose, possibly reducing it if you're experiencing hyperthyroid symptoms or increasing it if you have hypothyroid symptoms despite the lab values. During this waiting period, monitor for symptoms like fatigue, cold intolerance, weight changes, heart palpitations, or anxiety. The 4-6 week timeframe allows your body to reach a new steady state after dose adjustments, as levothyroxine has a long half-life of about 7 days. If you experience severe symptoms like significant heart palpitations, extreme fatigue, or mental status changes, contact your doctor immediately rather than waiting for the scheduled lab recheck. According to the most recent study 2, the effects of levothyroxine monotherapy versus combination therapy on quality of life and patient satisfaction are still being researched, but the current evidence does not support a consistent benefit of combination therapy. However, this does not directly impact the decision on when to recheck labs after a medication adjustment.
Some key points to consider:
- The American Thyroid Association, British Thyroid Association, and European Thyroid Association have discussed the use of combination therapy with levothyroxine and liothyronine, but the current evidence does not support its widespread use 3, 4.
- The safety of combination therapy has been reviewed, and it appears to be safe, but more research is needed to fully understand its effects 5.
- The decision to adjust the Synthroid dose should be based on the patient's symptoms and lab values, rather than solely on the lab values 6.
It is essential to work closely with your doctor to determine the best course of action and to monitor your symptoms and lab values regularly.