Thyroid hormones are combinations of the amino acid tyrosine and iodine. Two tyrosines are linked to either 1, 2, 3 or 4 iodine molecules to create what are known as T1, T2, T3 and T4, respectively.
T4 (levothyroxine) is the main thyroid hormone released from the thyroid gland, but is not considered significantly metabolically active. About 80% of thyroid hormone release from the thyroid gland is T4.
T3 (liothyronine) is considered the primary metabolically active thyroid hormone. This is the thyroid hormone that binds to receptors on the surface of cells and is transported within the cell to do what thyroid hormone does, namely turn on cellular energy.
Although some T3 is released by the thyroid gland, most of the T3 in the body is formed by conversion of T4 to T3. This is accomplished by an enzyme called “deiodinase” that removes one iodine from T4. Each tissue type in the body has it’s own deiodinase enzyme and they may behave differently thus leading to variable symptoms of low thyroid depending on which tissue type is most affected. The deiodinase enzyme in the peripheral body behaves quite differently than the enzyme in the brain, further causing variations in symptoms that may not correlate with standard thyroid testing. For more details on this see our article on Pitfalls of Standard Thyroid Tests.
T2 and T1 are present in small amounts and their exact function is uncertain.
Reverse T3 is a chemical mirror image of T3 and is also made from T4. It may bind to thyroid receptors but not activate them. If reverse T3 is relatively high is may create a low thyroid condition known as Reverse T3 Syndrome.
Thyroid hormones are poorly soluble in water, and more than 99% of the T3 and T4 circulating in blood is bound to carrier proteins, mainly thyroid binding globulin. This provides a stable pool of thyroid hormones from which the active, free hormones are released for uptake by target cells. Only the “free” portion of the thyroid hormone, as with other hormones, is biologically active. This is why we prefer to check free T4 and free T3 levels.
For more information on thyroid testing see Thyroid Tests.