Thyroid Gland

Today i was lucky to be able to attend a video lecture in my school streamed live from Oxford on a lecture about the thyroid gland and was able to glean many useful facts on this very important endocrine gland (linked back to my post on the endocrine system!)

The thyroid is an endocrine gland found in the very beginning of the trachea and is very significant as it is responsible for the production of hormones, tri-iodothyronine (T3) and thyroxine (T4), which are important for normal development and control of metabolic rate and thermogenesis. The thyroid is a unique organ as it is the only body organ that stores and solely uses iodine to produce T3 and T4 in order to control metabolism. T3 and T4 production is mainly regulated by external stimuli (such as cold, stress, etc) that activates the thyroid when a high metabolism is needed. The hypothalamus releases the thyroid releasing hormone which causes activation of the anterior pituitary which in turn releases a thyroid stimulating hormone causing the thyroid to produce T3 and T4 which then send back inhibitory feedback to stop this production, as overactivity is harmful resulting in hypothyroidism and hyperthyroidism.


So firstly what really is metabolic rate? Basal metabolic rate (BMR) is defined as the energy needed to maintain body temperature and body processes, and even if you do nothing all day the BMR is still quite high to regulate body temperature and reactions taking place in body cells. T3 acts on nearly every tissue and increases BMR by increasing O2 use and heat production through the increased Na/K (sodium potassium) pump. It can breakdown proteins, carbohydrates and lipids in order to release energy. It also increases cardiac output by increasing production of myosin and B1 receptors.

One of the problems of failure thyroid gland in producing T4 can result in hypothyroidism which results in low T4 levels and high TSH levels due to low negative feedback. This can often lead to poor neural development, stunted growth and muscle weakness, however it can be treated by injecting T4 (which has a long half life) to replenish and replace low T4 levels of the thyroid. This can also result due to iodine deficiency which is common in desert and mountain regions, failure of the thyroid to develop or failure of pituitary to produce TSH (or hypothalamus to produce TRH).

Another disorder of the thyroid is hyperthyroidism resulting in an overactive thyroid producing high T3 and T4 levels due to lack of negative feedback (to stop this production). It can result in loss of weight, intolerance to heat, hot hands, goitre, restlessness and anxiety and hand tremors (due to neuromuscular activity). Hyperthyroidism can be cured by injecting radioactive iodine into the body to destroy the thyroid and then injecting the body with T4 to replenish the lost supply of T4 due to the loss of the thyroid gland.