Thyroid Gland: The Anatomy, Functions and Its Hormones



The thyroid is a dual lobed gland located at the front of the neck, just beneath the larynx (voice box). One lobe of the thyroid is located on each side of the trachea (windpipe). The two lobes of the thyroid gland are connected by a narrow strip of tissue known as the isthmus. Some people do not have an isthmus, and instead, have two separate thyroid lobes.
As a component of the endocrine system, the thyroid gland secretes hormones that control important functions including metabolism, growth, heart rate, and body temperature. Found within thyroid tissue are structures known as parathyroid glands. These tiny glands secrete parathyroid hormone, which regulates calcium levels in the blood.

The thyroid gland is a butterfly-shaped organ located in the base of your neck. It releases hormones that control metabolism—the way your body uses energy. The thyroid’s hormones regulate vital body functions, including:
  • Breathing
  • Heart rate
  • Central and peripheral nervous systems
  • Body weight
  • Muscle strength
  • Menstrual cycles
  • Body temperature
  • Cholesterol levels
The thyroid gland is about 2-inches long and lies in front of your throat below the prominence of thyroid cartilage sometimes called Adam’s apple.

How does the Thyroid Gland Works: Hormone production and Regulations

The thyroid is part of the endocrine system, which is made up of glands that produce, store, and release hormones into the bloodstream so the hormones can reach the body’s cells. The thyroid gland uses iodine from the foods you eat to make two main hormones:

It is important that T3 and T4 levels are neither too high nor too low. Two glands in the brain—the hypothalamus and the pituitary communicate to maintain T3 and T4 balance.

Thyroid gland Regulation

Thyroid hormones T4 and T3 are regulated by the pituitary gland. This small endocrine gland is located in the middle of the base of the brain. It controls a multitude of important functions in the body. The pituitary gland is termed the “Master Gland” because it directs other organs and endocrine glands to suppress or induce hormone production. One of the many hormones produced by the pituitary gland is thyroid stimulating hormone (TSH).

The hypothalamus produces TSH Releasing Hormone (TRH) that signals the pituitary to tell the thyroid gland to produce more or less of T3 and T4 by either increasing or decreasing the release of a hormone called thyroid-stimulating hormone (TSH).

  • When T3 and T4 levels are low in the blood, the pituitary gland releases more TSH to tell the thyroid gland to produce more thyroid hormones.
  • If T3 and T4 levels are high, the pituitary gland releases less TSH to the thyroid gland to slow production of these hormones.
how the thyroid gland works


The reason Why You Need a Thyroid Gland in your body

T3 and T4 travel in your bloodstream to reach almost every cell in the body. The hormones regulate the speed with which the cells/metabolism work. For example, T3 and T4 regulate your heart rate and how fast your intestines process food. So if T3 and T4 levels are low, your heart rate may be slower than normal, and you may have constipation/weight gain. If T3 and T4 levels are high, you may have a rapid heart rate and diarrhea/weight loss.

Listed below are other symptoms of too much T3 and T4 in your body (hyperthyroidism):

  • Anxiety
  • Irritability or moodiness
  • Nervousness, hyperactivity
  • Sweating or sensitivity to high temperatures
  • Hand trembling (shaking)
  • Hair loss
  • Missed or light menstrual periods

The following are other symptoms that may indicate too little T3 and T4 in your body (hypothyroidism):

  • Trouble sleeping
  • Tiredness and fatigue
  • Difficulty concentrating
  • Dry skin and hair
  • Depression
  • Sensitivity to cold temperature
  • Frequent, heavy periods
  • Joint and muscle pain.

Thyroid Follicles and Thyroid Function

The thyroid is highly vascular, meaning that it has a wealth of blood vessels. It is composed of follicles that absorb iodine, which is needed to produce thyroid hormones. These follicles store iodine and other substances necessary for thyroid hormone production. Surrounding the follicles are follicular cells. These cells produce and secrete thyroid hormones into circulation via blood vessels. The thyroid also contains cells known as parafollicular cells. These cells are responsible for the production and secretion of the hormone calcitonin.

Thyroid Function

The primary function of the thyroid is to produce hormones that regulate metabolic function. Thyroid hormones do so by influencing ATP production in cell mitochondria. All cells of the body depend on thyroid hormones for proper growth and development. These hormones are required for proper brain, heart, muscle, and digestive function. In addition, thyroid hormones increase the body’s responsiveness to epinephrine (adrenaline) and norepinephrine (noradrenaline). These compounds stimulate sympathetic nervous system activity, which is important for the body’s flight or fight response. Other functions of thyroid hormones include protein synthesis and heat production. The hormone calcitonin, produced by the thyroid, opposes the action of parathyroid hormone by decreasing calcium and phosphate levels in the blood and promoting bone formation.

The following are commonly used thyroid tests

Measurement of Serum Thyroid Hormones: T4 by RIA  
T4 by RIA (radioimmunoassay) is the most used thyroid test of all. It is frequently referred to as a T7 which means that a resin T3 uptake (RT3u) has been done to correct for certain medications such as birth control pills, other hormones, seizure medication, cardiac drugs, or even aspirin that may alter the routine T4 test. The T4 reflects the amount of thyroxine in the blood. If the patient does not take any type of thyroid medication, this test is usually a good measure of thyroid function.

Measurement of Serum Thyroid Hormones: T3 by RIA
As stated on our thyroid hormone production page, thyroxine (T4) represents 80% of the thyroid hormone produced by the normal gland and generally represents the overall function of the gland. The other 20% is triiodothyronine measured as T3 by RIA. Sometimes the diseased thyroid gland will start producing very high levels of T3 but still produce normal levels of T4. Therefore measurement of both hormones provides an even more accurate evaluation of thyroid function.

Thyroid Binding Globulin
Most of the thyroid hormones in the blood are attached to a protein called thyroid binding globulin (TBG). If there is an excess or deficiency of this protein it alters the T4 or T3 measurement but does not affect the action of the hormone. If a patient appears to have normal thyroid function, but an unexplained high or low T4, or T3, it may be due to an increase or decrease of TBG. Direct measurement of TBG can be done and will explain the abnormal value. Excess TBG or low levels of TBG are found in some families as an hereditary trait. It causes no problem except falsely elevating or lowering the T4 level. These people are frequently misdiagnosed as being hyperthyroid or hypothyroid, but they have no thyroid problem and need no treatment.

Measurement of Pituitary Production of TSH
The pituitary production of Thyroid-stimulating hormone (TSH) is measured by a method referred to as IRMA (immunoradiometric assay). Normally, low levels (less than 5 units) of TSH are sufficient to keep the normal thyroid gland functioning properly. When the thyroid gland becomes inefficient such as in early hypothyroidism, the (TSH) thyroid-stimulating hormone becomes elevated even though the T4 and T3 may still be within the “normal” range. This rise in thyroid stimulating hormone (TSH) represents the pituitary gland’s response to a drop in circulating thyroid hormone; it is usually the first indication of thyroid gland failure.

Since Thyroid stimulating hormone is normally low when the thyroid gland is functioning properly, the failure of TSH(Thyroid stimulating hormones) to rise when circulating thyroid hormones are low is an indication of impaired pituitary function. The new “sensitive” Thyroid stimulating hormone test will show very low levels of TSH when the thyroid is overactive (as a normal response of the pituitary to try to decrease thyroid stimulation). Interpretations of the Thyroid stimulating hormone level depends upon the level of thyroid hormone; therefore, the TSH is usually used in combination with other thyroid tests such as the T4 RIA and T3 RIA.

TRH Test
In normal people TSH secretion from the pituitary can be increased by giving a shot containing TSH Releasing Hormone (TRH (thyroid Releasing Hormone): the hormone released by the hypothalamus which tells the pituitary to produce TSH). A baseline TSH of 5 or less usually goes up to 10-20 after giving an injection of Thyroid Releasing Hormone(TRH). Patients with too much thyroid hormone (thyroxine or triiodothyronine) will not show a rise in Thyroid stimulating hormone when given TRH. This “Thyroid Releasing Hormone test” is presently the most sensitive test in detecting early hyperthyroidism. Patients who show too much response to TRH (TSH rises greater than 40) may be hypothyroid. This test is also used in cancer patients who are taking thyroid replacement to see if they are on sufficient medication. It is sometimes used to measure if the pituitary gland is functioning. The new “sensitive” TSH test (above) has eliminated the necessity of performing a TRH test in most clinical situations.

Iodine Uptake Scan

A means of measuring thyroid function is to measure how much iodine is taken up by the thyroid gland (RAI uptake). Remember, cells of the thyroid normally absorb iodine from our blood stream (obtained from foods we eat) and use it to make thyroid hormone. Hypothyroid patients usually take up too little iodine and hyperthyroid patients take up too much iodine. The test is performed by giving a dose of radioactive iodine on an empty stomach.

The iodine is concentrated in the thyroid gland or excreted in the urine over the next few hours. The amount of iodine that goes into the thyroid gland can be measured by a “Thyroid Uptake“. Of course, patients who are taking thyroid medication will not take up as much iodine in their thyroid gland because their own thyroid gland is turned off and is not functioning.

At other times the gland will concentrate iodine normally but will be unable to convert the iodine into thyroid hormone; therefore, interpretation of the iodine uptake is usually done in conjunction with blood tests.


Goiters: Abnormally Large Thyroid Glands

A goiter is an abnormally large thyroid gland. A goiter develops either because the whole gland is swollen or the gland has multiple growths or nodules on it. While some people with a goiter have no symptoms, others may have symptoms of an overactive or underactive thyroid.


  • Hyperthyroidism is the medical term for an overactive thyroid, which produces too much thyroid hormone. Symptoms may include: Anxiety, rapid heart rate, diarrhea, weight loss.
  • Hypothyroidism refers to an underactive thyroid, which products too little thyroid hormone. Symptoms may include: Depression, fatigue, constipation, weight gain.
What Causes a Goiter?
Causes of goiters include:
  • Iodine deficiency — A goiter may be caused by not getting enough iodine through the foods you eat. However, it is rare in the United States, because table salt is supplemented with iodine.
  • Graves’ disease — This autoimmune disorder causes hyperthyroidism. Grave’s disease causes the body to produce a protein called thyroid-stimulating immunoglobulin that mistakenly attacks the thyroid, causing it to overproduce thyroid hormones and swell in size.
  • Hashimoto’s disease — This is another autoimmune disorder in which antibodies damage thyroid cells, leaving fewer cells to produce thyroid hormones. The pituitary gland, which controls your thyroid, stimulates the thyroid to produce more hormones, making the thyroid swell. Hashimoto’s disease is the most common cause of hypothyroidism.


  • Thyroid nodules — Nodules are over growths of tissue that may overproduce thyroid hormone or many not cause any symptoms. Rarely, nodules may contain cancer cells.
  • Thyroiditis — This condition is an inflammation of the cells in the thyroid that may cause the thyroid to produce too much or too little thyroid hormone.
  • Thyroid cancer — Cancerous cells may grow in nodules on the thyroid.

Treatments for Goiters
Treatment depends on the cause of the goiter and your symptoms. If your goiter is caused by low iodine intake, you may be treated with iodine supplements and thyroid hormone.

If your goiter is caused by Hashimoto’s thyroiditis, and you have hypothyroidism, treatment typically involves taking a pill containing thyroid hormone every day.

If the goiter is caused by hyperthyroidism, you may be treated with radioactive iodine, which is given as a pill and causes the thyroid gland to shrink and make less thyroid hormone. Radioactive iodine is only absorbed by the thyroid gland, so it does not harm other cells in your body. Another treatment involves the removal of part or the entire thyroid by a surgeon. You may need to take thyroid hormones after surgery.

If the goiter does not cause any symptoms, you may not need any treatment. Your doctor will monitor the goiter over time and check to see if you develop symptoms of hypothyroidism or hyperthyroidism. If the goiter keeps growing and causes breathing or swallowing problems, your doctor may suggest that you have all or part of the thyroid gland removed by an endocrine surgeon.

Other thyroid diseases





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