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.
The conversion of Thyroxine (T4) to Triiodothyronine (T3) is done by deiodination in peripheral tissues, which includes the liver, kidney, and other tissues. Besides Levothyroxine (levothyroxine), the conversion is facilitated by monodeiodination. 1
From the Research
The most effective way to convert Thyroxine (T4) to Triiodothyronine (T3) besides Levothyroxine is by using a combination of Levothyroxine and sustained-release Liothyronine, as evidenced by a recent study in 2023 2. This approach has been shown to increase the serum T3/T4 ratio and minimize excursions in serum T3 concentration during 24 hours. The study suggests that a higher ratio of SR-T3 to LT4 than previously recommended may be necessary to effectively incorporate sustained-release T3 in the management of hypothyroidism.
Some key points to consider:
- Selenium is an essential nutrient for the deiodinase enzymes that transform T4 to T3, with a typical dose of 200 mcg daily.
- Zinc (15-30 mg daily) also supports thyroid function and T4 to T3 conversion.
- Liothyronine (Cytomel), a synthetic T3, can be used at doses of 5-25 mcg daily, often divided into multiple doses.
- Natural desiccated thyroid (NDT) medications like Armour Thyroid or Nature-Throid contain both T4 and T3 and are typically dosed at 1-3 grains (60-180 mg) daily.
- Tyrosine, an amino acid precursor to thyroid hormones, can be supplemented at 500-1500 mg daily.
- Vitamin B12, vitamin D, and magnesium also support optimal thyroid function.
It's worth noting that the conversion of T4 to T3 is crucial, as T3 is the active form of thyroid hormone that can enter cells and affect metabolism, while T4 is primarily a prohormone that must be converted to T3 to exert its effects. Stress management and addressing adrenal health are also important, as cortisol can inhibit T4 to T3 conversion. A study in 2012 3 highlighted the importance of considering localized brain hypothyroidism and the potential benefits of adding liothyronine to replacement therapy. However, the most recent and highest-quality study 2 provides the strongest evidence for the use of combined Levothyroxine and sustained-release Liothyronine.