What about T2 and T1?
These two other thyroid hormones aren't measured. However, they have been found to have a stimulatory effect on the activity of the enzyme that converts T4 to T3; in other words, they help in the vital conversion process. T2 is also effective in increasing liver metabolism, and the metabolism of the heart, muscle tissue and brown adipose tissue ("brown fat", the fat that is burned away rather than stored in the body).
Calcitonin is another thyroid hormone, produced and released by the C-cells. Along with parathyroid hormone (PTH), calcitonin regulates calcium.
Unlike the standard synthetic medical treatment for hypothyroidism, desiccated thyroid supplement (natural thyroid hormone taken from animal glands) contains T1, T2, T3, T4 and calcitonin.