Your thyroid is a small bowtie or butterfly-shaped gland, located in your neck, wrapped around the windpipe, behind and below the Adam's Apple area. The thyroid produces several hormones, of which two are key: triiodothyronine (T3) and thyroxine (T4). These hormones help oxygen get into cells, and make your thyroid the master gland of metabolism.
The thyroid has the only cells in the body capable of absorbing iodine. The thyroid takes in iodine, obtained through food, iodized salt, or supplements, and combines it with the amino acid tyrosine. The thyroid then converts the iodine/tyrosine into the hormones T3 and T4. The "3" and the "4" refer to the number of iodine molecules in each thyroid hormone molecule.
When it's in good condition, of all the hormone produced by your thyroid, 80% will be T4 and 20% T3. T3 is considered the biologically more active hormone -- the one that actually functions at the cellular level -- and is also considered several times stronger than T4.
Once released by the thyroid, the T3 and T4 travel through the bloodstream. The purpose is to help cells convert oxygen and calories into energy.
As mentioned, the thyroid produces some T3. But the rest of the T3 needed by the body is actually formed from the mostly inactive T4 by a process sometimes referred to as "T4 to T3 conversion." This conversion of T4 to T3 can take place in some organs other than the thyroid, including the hypothalamus, a part of your brain.
The thyroid is part of a huge feedback process. The hypothalamus in the brain releases Thyrotropin-releasing Hormone (TRH). The release of TRH tells the pituitary gland to release Thyroid Stimulating Hormone (TSH). This TSH, circulating in your bloodstream, is what tells the thyroid to make thyroid hormones and release them into your bloodstream.
