From the FDA Drug Label
The general aim of therapy is to normalize the serum TSH level TSH may not normalize in some patients due to in utero hypothyroidism causing a resetting of pituitary-thyroid feedback. 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 In adult patients with primary hypothyroidism, monitor serum TSH levels after an interval of 6 to 8 weeks after any change in dosage. Overtreatment or undertreatment with levothyroxine sodium tablets may have negative effects on growth and development, cardiovascular function, bone metabolism, reproductive function, cognitive function, gastrointestinal function, and glucose and lipid metabolism in adult or pediatric patients
Having low TSH and high T4 while on Synthroid (Levothyroxine) may indicate overtreatment. This is because the goal of levothyroxine therapy is to normalize the serum TSH level, and low TSH with high T4 suggests that the patient may be receiving too much medication. Overtreatment can have negative effects on various bodily functions, including cardiovascular function, bone metabolism, and cognitive function 1. It is essential to monitor serum TSH levels regularly and adjust the dosage as needed to avoid overtreatment 1.
From the Research
If you have low TSH and high T4 while taking Synthroid (levothyroxine), your dose is likely too high and should be reduced to prevent potential long-term complications such as osteoporosis or heart problems, as suggested by recent evidence 2. This situation indicates overtreatment of hypothyroidism, where the excess thyroid hormone from Synthroid is suppressing your TSH production while elevating T4 levels, which can lead to symptoms of hyperthyroidism like anxiety, rapid heartbeat, and weight loss. The goal is to achieve normal TSH and T4 levels, which usually resolves symptoms and maintains overall health.
- Typically, your doctor will lower your Synthroid dose by 12.5-25 mcg and recheck your thyroid levels in 6-8 weeks, as the half-life of levothyroxine is long and dose adjustments should be made cautiously 3.
- Do not adjust the dose on your own, as this can lead to further complications.
- Continue taking your current dose until you speak with your doctor, and be prepared to discuss any symptoms you're experiencing and bring your most recent lab results to your appointment.
- Recent studies have shown that maintaining normal TSH levels is crucial in reducing the risk of death in hypothyroid patients treated with levothyroxine, with an estimated reduction in mortality risk of 28.3 million people in the USA and Europe alone 2.
- The stability of TSH levels in patients treated with different levothyroxine formulations, such as liquid or tablet, is also an important consideration, with some studies suggesting that liquid formulations may be more effective in maintaining normal TSH levels in the long-term 4.