From the FDA Drug Label
The general aim of therapy is to normalize the serum TSH level In adult patients with primary hypothyroidism, monitor serum TSH levels after an interval of 6 to 8 weeks after any change in dosage. In patients with hypothyroidism, assess the adequacy of replacement therapy by measuring both serum TSH and total or free-T4.
Levothyroxine dosing is based on TSH levels because the goal of therapy is to normalize the serum TSH level. TSH is a more sensitive indicator of thyroid hormone levels than T4, as it can detect subtle changes in thyroid function.
- TSH levels are used to monitor the adequacy of replacement therapy in patients with hypothyroidism.
- T4 levels are also measured, but the primary goal is to normalize the TSH level. 1
From the Research
Levothyroxine (Synthroid) dosing is primarily based on TSH levels rather than T4 levels because TSH provides a more sensitive and accurate reflection of thyroid hormone activity at the tissue level. When adjusting levothyroxine dosage, clinicians target TSH values within the normal range (typically 0.4-4.0 mIU/L), with more specific targets depending on the patient's age and condition 2. While T4 levels are important, TSH functions as an amplified feedback signal that responds to even small changes in thyroid hormone levels. This relationship makes TSH a more reliable indicator of whether the body is receiving appropriate thyroid hormone replacement.
Some of the key factors to consider when determining the optimal levothyroxine dose include:
- The patient's weight, with a common starting dose of approximately 1.6 μg/kg 3
- The patient's age and condition, with more specific targets depending on these factors 4
- The presence of other medical conditions, such as gastrointestinal disorders, that may affect levothyroxine absorption 3
- The use of other medications, such as calcium supplements or proton-pump inhibitors, that may interact with levothyroxine 3
Additionally, TSH correlates better with clinical symptoms and long-term outcomes than T4 levels alone. The pituitary gland, which produces TSH, essentially acts as a sensitive "thermostat" for thyroid function, increasing TSH production when thyroid hormone levels are insufficient and decreasing it when levels are excessive. This biological feedback mechanism makes TSH the preferred monitoring parameter for determining if a patient's levothyroxine dose is providing adequate replacement therapy.
It's worth noting that some studies have investigated the role of reverse T3 (rT3) in patients with hypothyroidism on different thyroid hormone replacement therapies 2. However, the current evidence suggests that TSH remains the most reliable indicator of adequate replacement therapy.
In terms of specific dosing recommendations, a study published in 2013 found that a weight-based formula (1.5-1.3 μg/kg) was effective in calculating levothyroxine doses for postsurgical hypothyroidism 5. However, more recent studies have emphasized the importance of individualized dosing and regular monitoring of TSH levels to ensure optimal therapy 4, 3.
Overall, the most recent and highest quality evidence suggests that TSH levels should be used as the primary guide for adjusting levothyroxine dosage, with a goal of achieving a TSH level within the normal range (typically 0.4-4.0 mIU/L) 2.