How does thyroid gland function?
Thyroid gland secretes 2 hormones- Thyroxine (T4) and Triiodothyronine (T3). These hormones are a combination of Iodine+Amino acid (thyroglobulin). T3 ( it contains 3 iodine atoms ) is the active form of thyroid gland which is generated at its site of action by deiodination of inactive T4 (it contains 4 iodine atoms).
The conversion of T4 to T3 happens in the liver and gut and is dependent on selenium.
The levels of the thyroid hormone are regulated by the circulatory levels of Thyroid Stimulating Hormone (TSH) released by the pituitary gland. The TSH levels increase or decrease based on the levels of plasma T4 and T3 in circulation, externally it is also influenced by stress/cold/warmth.
Once the hormones are released by the thyroid gland it enters the bloodstream. Approximately 99.8% of T4 and T3 remain bonded to protein, these proteins are produced by the liver and they play an important role in uniform distribution of the hormones. The free form of T4 and T3 are the active forms that are responsible for all the bodily functions.
Iodine is an essential raw material for thyroid hormone production. Dietary iodine is absorbed in the gut and enters the circulation. During the production of thyroid hormone iodine is taken up from circulation. This uptake of iodine requires minerals like sodium, potassium and chloride. Once inside the gland iodine conjugates with amino acid to form T3 and T4 hormones. The minimum daily iodine intake that is essential for normal thyroid function is 150µg/day. Iodine deficiency and iodide excess both inhibit thyroid function. The thyroid gland secretes about 80 µg (103 nmol) of T4 and 4 µg (7 nmol) of T3 per day.
Effects of thyroid hormone in the body
T3, the active hormone, acts on special receptors found on the nucleus of your cells that bring the change in the DNA leading to various metabolic reactions in body:
Calorigenic Action: Thyroid hormone increases the oxygen consumption by all metabolically active tissues. In other words, it regulates the speed with which your body cells work, thereby resulting in the breakdown of fat and protein and reduction in body weight.
Effect on Heart: Thyroid hormone increases the cardiac output by increasing the heart rate.
Effect on muscles: Increases the contractility of muscles. It is essential for overall growth and skeletal maturation
Effect on carbohydrate metabolism: Thyroid hormone increases the rate of absorption of carbohydrates from the intestine. It enhances the glucose uptake by cells. It also decreases cholesterol levels in the blood
Effect on body temperature: Thyroid hormone increases the basal metabolic rate of the cells, thereby increases the temperature of the body. Excess of the hormone is related to excessive sweating.
Effect on Gastrointestinal tract: Thyroid hormone increases appetite and food intake. It also increases gastric motility. Hence excess of the hormone causes diarrhoea and lack of it causes constipation.
Effect on nervous system: Thyroid hormone improves mentation and memory. They have a stimulatory effect on the nerves
Effect on sex hormones: Thyroid hormone regulates fertility in both men and women. It is also essential for a healthy menstrual cycle in women.
Effect on vitamin metabolism: Thyroid hormones are involved in various cellular activities that demand vitamins.
Thyroid dysfunctions can be broadly categorized as below:
There are many thyroid related problems one can experience owing to the complex physiology of thyroid gland. They are classified as below:
Autoimmune thyroid disorders: When the body starts making antibodies that target the thyroid tissue, the condition is referred to as Autoimmune thyroid disorders. Most common is Hashimoto’s disease. Grave’s disease is another autoimmune condition of the Thyroid. Thyroid resistance: When there is failure in uptake of thyroid hormone at cells receptor site due to localized infection or inflammation.
Elevated thyroid binding protein: Most commonly seen in women with excess estrogen production or women on birth control pills. This reduces the amount of free thyroid hormone (active form)
Thyroid under-conversion: Also known as T3 syndrome, common during chronic stress, there is failure in conversion of T4 to T3 hormone which is the active form of the hormone.
Thyroid over-conversion: This pattern is sometimes seen in people with excess testosterone production, insulin resistance, type 2 diabetes and PCOS. Excess of T3 overwhelms the receptors thereby making them resistant to T3.
How do we diagnose thyroid dysfunction?
Dysfunction of the thyroid gland impacts the entire body and to identify the root cause a thorough investigation is must.
How do we do that?
Analyze the signs and symptoms, detailed history documentation
Advising the appropriate blood tests
Levels of TSH, T3, T4, free T3 and free T4
Thyroid Antibodies like Thyroid Peroxidase antibody (TPOAb), Thyroid receptor antibodies (TRAb), Thyroglobulin antibodies (TG)
Check for complete nutrition panel (vitamins & minerals including selenium & iodine)
Heavy metals are thyroid receptor disruptors & common cause of autoimmunity. They should be evaluated.
Check for infections (complete CBC, viral/fungal/parasitic infections) & inflammatory markers.
Imaging tests: like ultrasound or thyroid scan is done to identify nodules
While addressing any thyroid dysfunction, one should look at the whole body and shouldn’t treat only the thyroid symptoms. It is important to rule out autoimmunity, nutritional imbalances, infections, heavy metals etc and address the dysfunction from the roots.
Barrett, Kim E, and William F. Ganong. Ganong's Review of Medical Physiology. New York: McGraw-Hill Medical
Hole, J. W., Shier, D., Butler, J., & Lewis, R. (1998). Hole's essentials of human anatomy & physiology. Boston. Mass: WCB/McGraw-Hill.
Amino, N., 1988. 4 Autoimmunity and hypothyroidism. Baillière's Clinical Endocrinology and Metabolism, 2(3), pp.591-617.