The thyroid gland takes iodine from the food we eat and combines it with the amino acid tyrosine to make two thyroid hormones: thyroxine (T4) and triiodothyronine (T3). These hormones are released into the bloodstream and transported throughout the body to regulate metabolism.

T4 is the inactive form of thyroid hormone; it’s produced solely by the thyroid gland. T3 is the active form. The thyroid gland produces about 20% of the T3 in our bodies. Proper thyroid function requires the conversion of T4 to T3 by removing an iodine atom by deiodinase enzymes in the body. Most people have enzymes called deiodinases in organs like the liver, brain, kidneys, and heart, which produce the other 80% by converting T4 into T3, as needed. It is important that T3 and T4 levels are not too high or too low.

Production and release of T3 and T4 are controlled by the hypothalamus, pituitary gland, and thyroid gland. In a healthy body they create a feedback loop that maintains appropriate circulating hormone levels. The amount of T4 produced by the thyroid gland is regulated by the thyroid stimulating hormone (TSH). Transport of these hormones into tissue cells is also critical for activity.

Many factors can impair the conversion of T4 to T3, such as high or low levels of vitamins, minerals, and hormones. This may result in hypothyroidism. Low thyroid or hypothyroidism is a common condition, and many people who have it can take a commercial prescription hormone supplement that contains standardized amounts of T4 and/or T3. These can work well if the patient’s hormone levels remain stable. However, some patients need special strengths or ratios of T3 and T4. There are others whose hormone levels tend to fluctuate regularly, and they need medication that responds to changing needs. Compounding allows for customized dosages and easy dose adjustments based on current test results.