The thyroid is a small gland that lies in the neck about the level of the Adam’s apple and weighs approximately one ounce.  It produces thyroid hormone and calcitonin.  The parathyroid glands are very small and lie on the outside portion of the thyroid gland and secrete parathyroid hormone.  Thyroid hormone ignites your metabolism so you can burn fat and produce energy.  We will be focusing on thyroid hormone.

The thyroid gland is stimulated to make thyroid hormone by thyroid-stimulating hormone (TSH) which is produced in the pituitary gland located in the brain.  The pituitary is controlled by the hypothalamus in the brain which monitors the amount of circulating thyroid hormone.  Iodine must enter the thyroid gland through a transport system that is repaired with the intake of vitamin C.  There is usually about 20-30 mg of iodine in the body and 75 percent of it is stored in the thyroid.  In addition to iodine, iron, tyrosine, selenium, vitamin E, vitamin C, vitamin D, magnesium, zinc, copper, and vitamins B2, B3, and B6 are required for thyroid hormone production.

The thyroid gland produces two thyroid hormones: T4 (thyroxine) and T3 (triiodothyronine).  Eighty percent of thyroid hormone produced is T4 and twenty percent is T3.  T3 is ten times more biologically active than T4 and it is produced as a result of one iodine being cleaved from T4.  T4 is inactive so the majority of thyroid hormone produced is actually inactive.  The numbers “3” and “4” indicate the number of iodines.  This is key in understanding optimal thyroid function.  Both T4 and T3 are bound to proteins in the blood until they reach your cells and become unbound to work their magic on metabolism.

Most of the T4 is converted into T3 in the liver.  Approximately sixty percent of the T4 is converted into T3, twenty percent is converted into an inactive form of thyroid hormone known as reverse T3 (irreversible), and the remaining twenty percent is converted into T3S (T3 sulfate) and T3AC (triiodothyroacetic acid).

Reverse T3 can be problematic; even though it is inactive, it will still bind to T3 receptors and block T3 from binding and working its magic on metabolism.  Too much or too little cortisol that is produced by the adrenal glands will increase circulating levels of reverse T3.  This mechanism is due to suppressed liver detoxification and clearance of reverse T3 from excess cortisol production.  Stress can not only cause signs of hypothyroidism but it will also impair the liver’s ability to detoxify.  Cortisol will also suppress TSH production resulting in low thyroid function.  Immune system activation, high adrenaline, excess free radicals, aging, fasting, stress, prolonged illness, and diabetes will also drive the inactivation of T3 to reverse T3.

T3 and reverse T3 can also be inactivated by conversion into a hormone known as T2.  Elevated insulin levels due to a diet high in refined carbohydrates will also increase reverse T3 levels.  Toxic metals including mercury, cadmium and lead will also increase reverse T3 production.  T3S and T3AC are inactive until they are catalyzed by an enzyme in the GI tract known as sulfatase.  This enzyme is dependent on healthy gut bacteria.

Thyroid hormone’s main role is to control metabolism (energy production) inside the cell.  Our cells contain tiny factories called mitochondria that produce energy from fat, sugar and protein.  Thyroid hormone controls the function of the mitochondria which determines how much energy is produced.  Symptoms of low thyroid function are related to a decrease in energy production including:

Fatigue
Weight gain/inability to lose weight
Constipation
Dry/itchy skin
Dry brittle hair and nails
Depression
Headaches
Overly sensitive to cold