The thyroid is mostly composed of or made up of an element called "iodine". This iodine is produced and made available in the thyroid gland through cells called hormones, each containing a certain number of iodine molecules within them. These thyroid hormones are then sent from the thyroid gland, throughout the body, to regulate the metabolism or the rate at which the body functions. The two major thyroid hormones are the "T-4" and the "T-3" and blood levels of these are tested to detect abnormally low or abnormally high levels of them, caused by thyroid hormone imbalance-disorders.

Another major hormone is also blood tested for to evaluate thyroid function, called "TSH" or Thyroid Stimulating Hormone. This one comes from the pituitary gland so is not actually a thyroid hormone but the master gland in the brain that monitors and regulates the thyroid and the blood TSH level gives an accurate reflective-measure of how well the thyroid is functioning, in producing its own hormones. Adversely, TSH is low when the thyroid is overactive and high when it is under-active, while the T-4 and T-3 will correlate as low with an under-active thyroid and high when the thyroid is overactive.

The T-4 hormone is the thyroid hormone that contains four iodine molecules. Another name for the T-4 hormone is "Thyroxine" and it is more abundant or found in larger supply in the body than is the T-3 hormone, also called "Triiodothyronine", which only contains three iodine molecules. Despite the fact that T-4 contains one more iodine molecule, T-3 is at least five times more powerful than T-4. The T-4 hormone is the less active (metabolically inactive) of the two and is stored in the body and converted into the more active (metabolically active) T-3 hormone, as it is needed in the body. This conversion process is accomplished via enzymes or proteins found in the body which attach to the T-4 hormone cells, turning them into T-3 hormone, as the body needs it to regulate metabolism. This conversion process takes place within the liver and partially within the kidneys as well.
When there is excess T-4 in the body, it is not converted into T-3 but instead is converted into a substance called "Reverse T-3". This is how the body rids itself of extra T-4 in the body that is in excess to the amount needed to be stored and converted into T-3. Reverse T-3 is a term that actually means that the excess T-4 hormone has been rendered inactive, so that too much of it is not converted into T-3 which would cause an overactive metabolism or a type of "hyperthyroidism" (overactive thyroid). Certain types of illnesses and severe, chronic stress, can cause the body to convert too much of the T-4 hormone into Reverse T-3, which will cause a person to experience a type of "hypothyroidism" (under-active thyroid), due to low T-3 in the body. This type of hypothyroidism that is secondary to an illness or stress in the body is also referred to as "Sick Euthyroid Syndrome" and is also sometimes referred to as "Wilson's Temperature Syndrome" and is usually a temporary form of hypothyroidism that can be corrected with short term T-3 hormone replacement therapy. This type of hypothyroidism is rare compared to the types that are caused by a diseased thyroid gland or "primary hypothyroidism" and is also rare compared to the types caused by disease or dysfuntioning of the glands that help regulate thyroid function, which is referred to as "Secondary Hypothyroidism" or "Central Hypothyroidism".