what is a thyroid

The thyroid is a butterfly shaped gland that sits in front of the windpipe, midway between the thyroid cartilage (“Adam’s apple”) and the top of the sternum.
In the average adult it weighs about 20 grams. It synthesises and secretes several hormones, two of which are extremely important: triiodothyronine (T3) and thyroxine (T4). These help oxygen get into cells, and make your thyroid the master gland of metabolism.
Thyroid hormone acts on many different cell types within the body.  The thyroid has the only cells in the body capable of absorbing iodine. It takes in iodine (obtained through food, iodised 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.)
80% of what the thyroid makes is T4, which is a stable compound that the cells convert into useable T3 as needed. However, 15% of the T3 your body needs comes directly from the thyroid gland.  Some of your cells demand serum T3 straight from the thyroid; they cannot convert T4 into 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. Other hormones that a healthy thyroid gland produces are T0, T1, T2 and Calcitonin.  Calcitonin is the hormone which keeps the calcium in your bones. Research is ongoing into the functions of the other T hormones. 
Once released by the thyroid, the T3 and T4 travel through the bloodstream. Their purpose is to help cells convert oxygen and calories into energy.
The remaining T3 needed by the body is formed from the mostly inactive T4, by a process sometimes referred to as "T4 to T3 conversion."  This conversion takes place in the liver and  kidneys.
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).
Circulating in your bloodstream, TSH tells the thyroid to make and release thyroid hormones.