From the FDA Drug Label
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. Approximately 80% of circulating T3 is derived from peripheral T4 by monodeiodination Approximately 80% of the daily dose of T4 is deiodinated to yield equal amounts of T3 and reverse T3 (rT3)
The conversion of Thyroxine (T4) to Triiodothyronine (T3) occurs through deiodination in peripheral tissues, with approximately 80% of T3 being derived from T4. This process involves the monodeiodination of T4 to produce T3. 1
From the Research
Thyroxine (T4) is converted to Triiodothyronine (T3) primarily in peripheral tissues through the action of enzymes called deiodinases, with the most recent evidence suggesting that this conversion is crucial for maintaining normal thyroid function and overall health 2. The conversion of T4 to T3 is essential because T3 is the more active thyroid hormone that binds to nuclear receptors and regulates metabolism.
- The main enzyme responsible is type 1 deiodinase (D1), which removes one iodine atom from the outer ring of T4 to produce T3.
- This process occurs mainly in the liver, kidneys, and thyroid gland.
- Type 2 deiodinase (D2) also converts T4 to T3, primarily in the brain, pituitary, and brown adipose tissue. Several factors can affect this conversion, including:
- Nutritional status
- Illness
- Medications
- Stress Selenium is a crucial micronutrient for deiodinase function, as these enzymes are selenoproteins. Certain medications like propylthiouracil, high-dose propranolol, amiodarone, and glucocorticoids can inhibit T4 to T3 conversion. Additionally, conditions like severe illness, starvation, and chronic inflammatory states can decrease conversion, leading to what's known as "low T3 syndrome" or "euthyroid sick syndrome" 3. The most recent study on this topic, published in 2021, highlights the importance of considering the conversion of T4 to T3 in the management of hypothyroidism, and suggests that combination therapy with LT4 and T3 may be beneficial for some patients 2. However, it's worth noting that the evidence is not yet conclusive, and more research is needed to fully understand the role of T4 to T3 conversion in thyroid function and disease 4, 5.