your thyroid produces and secretes the T4 hormone, some is lost by protein, then what is left is known as Free T4 is converted mostly by your liver is called T3, part of that is also lost to protein, then the remainder available
for use by your cells is called Free
T3, when the
T3 is low, the Pituitary Gland located in the brain senses this low level, it sends its own hormone called Thyroid Stimulating Hormone (TSH) to the Thyroid to "Bump" it or Stimulate it into producing more T4, s