From the FDA Drug Label
Thyroid hormones exert their physiologic actions through control of DNA transcription and protein synthesis. Triiodothyronine (T3) and L-thyroxine (T4) diffuse into the cell nucleus and bind to thyroid receptor proteins attached to DNA. The physiological actions of thyroid hormones are produced predominantly by T3, the majority of which (approximately 80%) is derived from T4 by deiodination in peripheral tissues.
The main difference between T3 and T4 is that:
- T3 is the more active form of thyroid hormone, responsible for the majority of physiological actions.
- T4 is converted to T3 in peripheral tissues, with approximately 80% of T3 derived from T4. In simple terms, T4 is like a precursor to T3, and T3 is the form that actually produces the effects of thyroid hormones in the body 1.
From the Research
T3 and T4 are two different thyroid hormones, with T4 being the main hormone produced by the thyroid gland and T3 being the more active form that directly affects cells and controls metabolic rate, heart function, brain development, and body temperature. The key difference between T3 and T4 lies in their function and potency. T4 is considered a precursor or "storage form" that gets converted into T3 when needed, while T3 is the active form that directly influences cellular activity. Some of the main points to consider when discussing T3 and T4 include:
- T4 is the primary hormone produced by the thyroid gland, while T3 is produced in smaller amounts and is more potent [ 2 ].
- T4 is converted to T3 in the body, and this conversion is essential for the proper functioning of thyroid hormones [ 3 ].
- The balance between T3 and T4 is crucial for maintaining normal energy levels, weight management, and overall health [ 4 ].
- Some thyroid medications contain T4 only, while others contain T3 or a combination of both [ 5 ]. It's essential to note that the current guideline-based management of hypothyroidism recommends monotherapy with levothyroxine (LT4), titrated to maintain the level of thyrotropin within a euthyroid reference range [ 3 ]. However, some studies suggest that combination therapy with LT4 and T3 may be beneficial for certain patients, particularly those with residual symptoms despite thyrotropin-optimized LT4 [ 2 ]. Ultimately, the decision to use T4, T3, or a combination of both should be based on individual patient needs and preferences, and should be made in consultation with a healthcare professional [ 5 ].