What is normal thyroid levels?

                  The hormones produced by the thyroid are thyroxine (T4) and triiodothyronine (T3).  Both are conjugates of iodine, proteins, and the amino acid tyrosine and they regulate the body’s metabolic rate.  Their production is stimulated by the release of thyroid stimulating hormone (TSH) from the pituitary gland.  If T4 and T3 levels drop too low, TSH is released in the blood and their production rate in increased.  Once there is a sufficient level of T3 and T4, TSH release is inhibited.  While T4 can remain active in the body for over a month, T3 has a half life of about two days and is then broken down in the liver.  The release of thyroxine causes the production and increased action of mitochondria, which are the power plants of cells.  This directly increases the metabolic rate and the rate of protein production and breakdown.  It also acts on the central nervous system, increasing the activity of the entire endocrine system.  Normal levels of T4 in the body are between 5 to 14 mcg/dL.  Any levels outside this range are characteristic of hypo- or hyperthyroidism, which will be discussed later.  Triiodothyronine is actually a derivative of thyroxine.  It is mainly produced by the removal of an iodine molecule from T4, which can occur in the liver or the kidneys but it can also be produced directly in the thyroid, where it is stored until release.  T3 is also responsible for the regulation of the metabolic rate but it is a much more effective hormone than T4.  Normal T3 levels range from 80 to 230 ng/dL and any measurement outside of this range is again characteristic of thyroid abnormalities.