The thyroid gland is located in the front of the neck that straddles the trachea in a bow tie-like fashion. The thyroid gland can produce and store hormones and also contains the parathyroid glands within the thyroid tissue. The thyroid is associated with weight regulation, energy production and blood glucose use. Many things can happen to cause the thyroid to overreact to a stimulus or stop responding as well.
Feedback Mechanism
There are many feedback loops within the body to help keep the system in check. It is important to know about the feedback mechanism to get an idea of where any potential problems may be happening.
The 19th edition of "Review of Medical Physiology" explains that the feedback mechanism is obtained with three different organs and the blood. The hypothalamus is centered deep with in the brain and waits for the level of circulating hormones to drop before it sends a signal to the pituitary gland with thyroid releasing hormone (TRH). The pituitary gland is another part of the brain that is also responsible for hormone production. Once the pituitary gland receives a signal from the hypothalamus, it will send a production signal to the thyroid gland with thyroid stimulating hormone (TSH). The thyroid will then respond by producing more hormones. The level of circulating hormone in the blood will make its way back up to the brain where the hypothalamus is. Once there is enough thyroid hormone in the blood, the hypothalamus will stop producing TRH, until levels drop again.
Hypothyroid
In hypothyroidism, the output of thyroid hormones slow so that the body itself slows down. There are two chemical hormones--tri-iodothyronine (T3) and thyroxine (T4)--that the thyroid makes. When it stops making these hormones, the body will start to experience side effects of a slow metabolism. These effects are usually not dramatic and take time to notice.
Causes
There are several causes for hypothyroidism, but the Mayo Clinic states the main one is an autoimmune disorder in which the body begins to make antibodies that will attack its own tissue. It is not well understood why this happens. This type of disorder is called Hashimoto's thyroiditis.
Various tumors can also affect hormone output at the level of the hypothalamus, pituitary or thyroid itself. The tumors can cause each area of the body to function differently and not produce TRH, TSH, T3 or T4. Lab tests can help find where the tumor is and what hormones are being affected.
Being treated for hyperthyroidism, when the body produces too much T3 and T4, is another cause for hypothryoidism. Occasionally, someone may receive irradiation treatments for various cancers than can damage the thyroid and cause it to respond differently.
Less common causes for hypothyroidism are being born with a damaged or ill functioning thyroid. That issue will be more apparent with birth as growth rate will be affected. Other less common causes according to the Mayo clinic include developing an autoimmune disorder during pregnancy where the body attacks itself, having part of the thyroid removed during surgery and iodine deficiency. Iodine deficiency is less of a problem in developed countries since iodine has been added to table salt.
Risks
The most commonly affected group is women who are over 50, but both categories are risk factors on their own. The risks are higher if there is a family member who has an autoimmune disorder. Other risks include having radiation treatments to the neck or chest area for cancer, being given radioactive iodine for hyperthyroidism treatments, taking medications like lithium, which affect the production of thyroid hormones or having part or all of the thyroid removed.
References
- Mayo Clinic: Hypothyroidism
- "Review of Medical Physiology - 19th edition;" William F. Ganong; 1999
- National Istitues of Health: Hypothyroidism
