T4 is usually regarded as the base hormone, the inactive form. However there are research papers showing T4 receptors in our brains. Anyway the idea is that T4 has to be converted to the active form known as T3. T3 is much more potent than T4. There is also T2 and T1. Mainstream websites will just write off T2 and T1 as junk hormones, inert chemicals that do nothing. That sounds like rubbish to me, and to many other doctors who are genuinely interested. Turns out, they are active hormones too.
Most of our hormones, of all kinds, are bound to special proteins. So only a small percentage of our T4 is available to be converted to T3. The portion available is called Free T4. When it’s converted to T3, most of that is bound to a special protein, and the small, active portion is called Free T3.
The real names for these hormones are:
T4 – thyroxine
T3 – triiodothyronine
T2 – diiodothyronine
T1 – monoiodothyronine
When T3 is made by pharma drug firms, it’s called liothyronine. Oddly when T4 is made by pharma drug firms, it should be called levothyroxine. There can be confusion about how these terms are used.
“The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3).Thyroid cells are the only cells in the body which can absorb iodine. These cells combine iodine and the amino acid tyrosine to make T3 and T4. T3 and T4 are then released into the blood stream and are transported throughout the body where they control metabolism (conversion of oxygen and calories to energy).
Every cell in the body depends upon thyroid hormones for regulation of their metabolism. The normal thyroid gland produces about 80% T4 and about 20% T3, however, T3 possesses about four times the hormone “strength” as T4. . . . (illustration removed).
The thyroid gland is under the control of the pituitary gland, a small gland the size of a peanut at the base of the brain (shown here in orange). When the level of thyroid hormones (T3 & T4) drops too low, the pituitary gland produces Thyroid Stimulating Hormone (TSH)which stimulates the thyroid gland to produce more hormones. Under the influence of TSH, the thyroid will manufacture and secrete T3 and T4 thereby raising their blood levels.”