Does Tirosint Contain T3?
No, Tirosint does not contain T3 (triiodothyronine)—it contains only levothyroxine (T4), which is then converted to T3 in the body, primarily in the liver. 1
What Tirosint Actually Contains
- Tirosint is a brand formulation of levothyroxine sodium, which is synthetic L-thyroxine (T4) that is chemically identical to the T4 produced in the human thyroid gland 1
- The medication contains only T4 hormone with the empirical formula C₁₅H₁₀I₄NNaO₄ and does not include any T3 in its formulation 1
How T4 Converts to T3 in Your Body
- Approximately 80% of circulating T3 in the body is derived from peripheral conversion of T4 through a process called monodeiodination 1
- The liver is the primary site where T4 is converted to T3 through type 1 deiodinase (DIO1), a selenium-dependent enzyme 2
- This conversion process is ATP-dependent and requires selenium as a cofactor for proper functioning of the deiodinase enzymes 2
Clinical Implications for Patients on Levothyroxine
- Normal T3 levels can be achieved with levothyroxine (T4) therapy alone in most patients, as demonstrated in studies of patients who underwent total thyroidectomy and maintained normal T3 concentrations on T4 monotherapy 3
- However, patients treated with levothyroxine may have relatively lower serum T3 concentrations compared to the general population, though this does not necessarily require T3 supplementation for most patients 4
- The physiological actions of thyroid hormones are produced predominantly by T3, making the peripheral conversion of T4 to T3 essential for normal thyroid hormone action 2
When T3 Measurement May Be Relevant
- T3 testing can be helpful in highly symptomatic patients with minimal free T4 elevations when evaluating for thyrotoxicosis 5
- However, T3 measurement does not add meaningful information when assessing T4 over-replacement in hypothyroid patients, as normal T3 levels can be seen even in over-replaced patients 6
- Iron deficiency can impair the hepatic conversion of T4 to T3, potentially leading to relative T3 deficiency despite normal T4 levels 5, 2