How Your Thyroid Works
"A delicate Feedback Mechanism"
Your thyroid gland is a small gland, normally weighing less than one ounce, located in the front of the neck. It is made up of two halves, called lobes that lie along the windpipe (trachea) and are joined together by a narrow band of thyroid tissue, known as the isthmus.
The thyroid is situated just below your " Adams apple" or larynx. During development (inside the womb), the thyroid gland originates in the back of the tongue, but it normally migrates to the front of the neck before birth. Sometimes it fails to migrate properly and is located high in the neck or even in the back of the tongue (lingual thyroid), this is very rare. At other times it may migrate too far and ends up in the chest (this is also rare).
The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3).
Thyroid cells are the only cells in the body which can absorb iodine. These cells combine iodine and the amino acid tyrosine to make T3 and T4. T3 and T4 are then released into the blood stream and are transported throughout the body where they control metabolism (conversion of oxygen and calories to energy). Every cell in the body depends upon thyroid hormones for regulation of their metabolism. The normal thyroid gland produces about 80% T4 and about 20% T3, however, T3 possesses about four times the hormone "strength" as T4.
The thyroid gland is under the control of the pituitary gland , a small gland the size of a peanut at the base of the brain (shown here in orange). When the level of thyroid hormones (T3 & T4) drops too low, the pituitary gland produces Thyroid Stimulating Hormone (TSH) which stimulates the thyroid gland to produce more hormones. Under the influence of TSH, the thyroid will manufacture and secrete T3 and T4 thereby raising their blood levels. The pituitary senses this and responds by decreasing its TSH production. One can imagine the thyroid gland as a furnace and the pituitary gland as the thermostat. Thyroid hormones are like heat. When the heat gets back to the thermostat, it turns the thermostat off. As the room cools (the thyroid hormone levels drop), the thermostat turns back on (TSH increases) and the furnace produces more heat (thyroid hormones).
The pituitary gland itself is regulated by another gland, known as the hypothalamus (shown in our picture in light blue). The hypothalamus is part of the brain and produces TSH Releasing Hormone (TRH) which tells the pituitary gland to stimulate the thyroid gland (release TSH). One might imagine the hypothalamus as the person who regulates the thermostat since it tells the pituitary gland at what level the thyroid should be set.
hypo = too little
thyroidism = disease of the thyroid
Thus, hypo-thyroidism = a disease of too little thyroid activity.
There are two common causes of hypothyroidism. The first is a result of previous (or currently ongoing) inflammation of the thyroid gland which leaves a large percentage of the cells of the thyroid damaged (or dead) and incapable of producing sufficient hormone. The most common cause of thyroid gland failure is called autoimmune thyroiditis (also called Hashimoto's thyroiditis), a form of thyroid inflammation caused by the patient's own immune system. The second major cause is the broad category of "medical treatments". The treatment of many thyroid conditions warrants surgical removal of a portion or all of the thyroid gland. If the total mass of thyroid producing cells left within the body is not enough to meet the needs of the body, the patient will develop hypothyroidism. Remember, this is often the goal of the surgery in surgery for thyroid cancer. However, at other times, the surgery will be to remove a worrisome nodule, leaving half of the thyroid in the neck undisturbed. Sometimes (often), this remaining thyroid lobe and isthmus will produce enough hormone to meet the demands of the body. For other patients, however, it may become apparent years later that the remaining thyroid just can't quite keep up with demand. Similarly, goiters and some other thyroid conditions can be treated with radioactive iodine therapy . The aim of the radioactive iodine therapy (for benign conditions) is to kill a portion of the thyroid to  prevent goiters from growing larger or  producing too much hormone (hyperthyroidism). Occasionally, (often?) the result of radioactive iodine treatment will be that too many cells are damaged so the patient often becomes hypothyroid a year or two later. This is O.K. and usually greatly preferred over the original problem. There are several other rare causes of hypothyroidism , one of them being a completely "normal" thyroid gland, which is not making enough hormones because of a problem in the pituitary gland. If the pituitary does not produce enough Thyroid Stimulating Hormone (TSH) then the thyroid simply does not have the "signal" to make hormone, so it doesn't.
Symptoms of Hypothyroidism
- Weight gain or increased difficulty losing weight
- Coarse, dry hair
- Dry, rough pale skin
- Hair loss
- Cold intolerance (can't tolerate the cold like those around you)
- Muscle cramps and frequent muscle aches
- Memory loss
- Abnormal menstrual cycles
- Decreased libido
Each individual patient will have any number of these symptoms that will vary with the severity of the thyroid hormone deficiency and the length of time the body has been deprived of the proper amount of hormones. Some patients will have one of these symptoms as their main complaint, while another will not have that problem at all and will be suffering from a different symptom. Most will have a combination of a number of these symptoms. Occasionally, some patients with hypothyroidism have no symptoms at all, or they are just so subtle that they go unnoticed.
Note: Although this may sound obvious, if you have these symptoms, you need to discuss them with your doctor and probably seek the skills of an endocrinologist. If you have already been diagnosed and treated for hypothyroidism and you continue to have any or all of these symptoms, you need to discuss it with your physician. Although treatment of hypothyroidism can be quite easy in some individuals, others will have a difficult time finding the right type and amount of replacement thyroid hormone.
Hypothyroidism can often be diagnosed with a simple blood test. In some persons, however, its not so simple and more detailed tests are needed. Most importantly, a good relationship with a good endocrinologist will almost surely be needed.
Potential Dangers of Hypothyroidism
Because the body is expecting a certain amount of thyroid hormone, the pituitary will make additional thyroid stimulating hormone (TSH) in an attempt to entice the thyroid to produce more hormone. This constant bombardment with high levels of TSH may cause the thyroid gland to become enlarged and form a goiter (termed a "compensatory goiter"). Left untreated, the symptoms of hypothyroidism will usually progress. Rarely, complications can result in severe life-threatening depression, heart failure or coma.