The thyroid is a butterfly shaped endocrine gland located at the base of the neck. Its function is to produce and store hormones that control metabolism in the human body. It does this by absorbing iodine and converting it into the hormones T3 and T4. The thyroid is infamous for developing disorders and diseases, the most common of these being hyperthyroidism and hypothyroidism. These diseases are much more likely to develop in woman than in men. Often, these ailments cannot be cured, but they can often be treated with medications.
For many years, people have known of the thyroid gland. Ancient people would often treat thyroid goiters with seaweed and sometimes animal thyroid gland. In 150 A.D. Galen, a Roman figure of ancient medicine, suggested that the thyroid’s function was to lubricate the throat. Chinese physician Wang Hei gave the first description of the thyroid gland until modern times. The thyroid was named by Thomas Wharton in 1656. He named the thyroid after an ancient Greek shield, because he thought the thyroid resembled its shape. After Iodine was discovered from the ashes of burnt seaweed, it became a treatment for goiters. Theodor Kocher discovered the cause and cure for hypothyroidism in 1888 and was later awarded the Nobel Prize for Medicine. 
The thyroid gland is of a reddish-brown color and is located at the base of the neck. It is composed of two lobes connected by the thyroid isthmus. The thyroid weighs between twenty-five and thirty grams, and tends to be heavier in women. The thyroid increases in size during pregnancy and menstruation.
The thyroid is connected to the body by visceral fascia, a connective tissue. The visceral fascia connects to the laryngoskeleton of the larynx, which is responsible for moving the thyroid during swallowing. The thyroid is also connected to the cricoid cartilage and the thyroid cartilage
Blood is supplied to the thyroid from the superior and inferior thyroid arteries and the thyroid ima artery. Blood leaves the thyroid from the middle and inferior thyroid veins and the thyroid ima vein. 
The thyroid gland is made up of two types of specialized cells. The first of these cells are the thyroid epithelial cells. These cells create the thyroid hormones. The thyroid epithelial cells are arranged into spheres known as thyroid follicles. Thyroid follicles are filled with a colloid of thyroid hormones.
The second types of specialized cells in the thyroid are the paraifollicular cells or C cells. C cells produce the chemical, calcitonin.
The main function of the thyroid gland is to convert iodine into thyroid hormones. The thyroid’s cells, follicular cells, are the only cells in the human body equipped to absorb iodine. The thyroid absorbs the iodine and combines it with tyrosine amino acids. This produces the thyroid hormones triiodothyronine and thyroxine, also known as T3 and T4.  T3 and T4 are then released into the bloodstream where they travel to every cell in the body sending chemical messages to the cells to either speed up or slow down their processes. These processes are what controls metabolism. 
Another function of the thyroid gland is to produce a hormone known as calcitonin. Calcitonin is produced by the parafollicular thyroid cells. Calcitonin is a 32 amino acid peptide hormone that has many processes. It controls the activity of osteoclasts and the calcium level of the blood. 
The thyroid gland is the first endocrine gland to develop in a fetus. It begins to develop just 24 days after gestation. The thyroid begins to develop from the pharyngeal endoderm. As the fetus grows, the thyroid descends to the throat and attaches to the base of the tongue by the thyroglossal duct, which disappears as the fetus further develops.  After seven weeks of development, the thyroid is fully developed, and by twelve weeks, it begins to produce fetal hormones. 
Hyperthyroidism is a condition in which the thyroid produces too much of the thyroid hormones T3 and T4. Because the thyroid is producing too much of these hormones, the body's metabolism is sped up. There are many common symptoms of Hyperthyroidism. These symptoms include the following: increased heart rate, palpitations of the heart, intolerance to heat and excessive sweating, insomnia, fatigue, weight loss even with a normal appetite, muscle weakness and tremors, shortness of breath, hair loss, diarrhea, light or completely absent menstrual periods, and anxiety. 
Graves disease is the primary cause of hyperthyroidism. It is an autoimmune disease where the immune system produces antibodies that imitate the thyroid-stimulating hormone (TSH). This causes the thyroid gland to produce an excess of thyroid hormones, also known as hyperthyroidism. The exact cause of Graves' disease is unknown, but it is believed that a combination of genetics and environmental factors cause the disease.
De Quervain's Thyroiditis
De Quervain's Thyroiditis is a thyroid disease where the thyroid rapidly swells and becomes very painful. The thyroid secretes an excess amount of the thyroid hormone into the blood stream, resulting in temporary hyperthyroidism, which is often followed by hypothyroidism. Most people who obtain this disease will have a recovery time ranging from a few weeks to months. De Quervain's Thyroiditis is a rare, seasonal disease. 
Hypothyroidism is a condition in which the thyroid does not produce enough of the T3 and T4 hormones. This causes a significant slow down in the body’s metabolism. This condition is extremely common and is currently affecting over five million Americans. Common symptoms of hypothyroidism include the following: fatigue, brittle hair and nails, weight gain and difficulty in loosing weight, pale skin, heavy menstrual cycles, anxiety, depression, weakness, body pains, decreased bowel movements, loss of memory, and an intolerance to cold. 
Hashimoto's thyroiditis is an autoimmune disease in which the thyroid gland is attacked and destroyed by the body. The cause of this disease is still unknown. A person with Hashimoto's thyroiditis becomes hypothyroid when the thyroid is attacked by the body and can no longer produce a sufficient amount of thyroid hormones. Symptoms of Hashimoto's thyroiditis include all of the symptoms of hypothyroidism along with the following: increased cholesterol levels, a round face, puffy eyes, goiters, and a weakened immune system. Hashimoto's thyroiditis has no cure, but a person with the disease can take hormone replacements. If a person with a severe case of Hashimoto's thyroiditis does not receive treatment, they will become extremely ill. They may experience seizures, slip into a coma, or go into cardiac arrest. 
Postpartum thyroiditis is a harmless inflammation of the thyroid gland that sometimes occurs in woman four to twelve months after they have given birth. At first the woman will experience symptoms of hyperthyroidism, but will later experience hypothyroidism. The thyroid usually functions normally twelve to eighteen months after the condition develops, but for some woman the hypothyroidism does not go away and they must take hormone replacement pills for the rest of their lives. 
Hypothyroidism often occurs as a reaction to certain medical treatments. Hypothyroidism occurs if the thyroid receives treatment with radiation or if the thyroid is removed surgically.
Other factors that increase a person’s risk of contracting hypothyroidism are being over the age of 50, exposure of the neck during an x-ray, radiation treatments of any kind, obesity, and being a female. 
It is reported that there are about 20,000 new cases of thyroid cancer each year. There are four types of thyroid cancer and the most common of these cancers, papillary and follicular cancers, have an extremely high survival rate. Thyroid cancer occurs when a nodule appears on the thyroid and becomes cancerous. Many people develop nodules, but there is a 99% chance that they will not be cancerous. Symptoms of thyroid cancer are enlarged lymph nodes, a hoarse voice, and neck pain. Thyroid cancer is usually treated with the surgical removal of the thyroid and radioactive iodine. 
- Thyroid Disease 101: The Basics Mary Shomon, The New York Times Company, 2009.
- The History of the Thyroid Gland unknown author, EndocrineSurgeon.co.uk.
- The thyroid gland unknown author, Taylor and Frances, 2001.
- Fetal Thyroid Development and Congenital Abnormality Risks Christine Cadena, Associated Content, Inc. , December 03, 2007.
- Thyroid Anatomy Dominique Dorion, MD, MSc, FRCSC and David Lemaire, MD, Medscape, Mar 5 2008.
- Functional Anatomy of the Thyroid and Parathyroid Glands R. Bowen, February 15, 1999.
- Your Thyroid Gland James Norman, MD, FACS, FACE, Vertical Health, 2009.
- Exactly why is your thyroid so important? Jessika Toothman, Discovery Communications, 2008.
- Calcitonin R. Bowen, October 11, 2003.
- Are The Symptoms Of Hyperthyroidism? unknown author, Nucleus Medical Art, October 2004.
- Understanding Graves' Disease - the Basics John A. Seibel, WebMD, LLC, December 13 2008.
- De Quervain Thyroiditis Ildiko Lingvay, MD, MPH, Daniel Matei Brailita, MD, Joseph E Loewenstein, MD, James Burks, MD, FACP, FACE, and KoKo Aung, MD, MPH, FACP, Medscape, December 16 2008.
- Hashimoto's Thyroiditis Ruchi Mathur, MD, FRCP, MedicineNet, Inc., 2009.
- Postpartum depression Todd B. Nippoldt, M.D., mayoclinic.com, April 18 2007.
- Hypothyroidism Elizabeth H. Holt, MD, PhD, medlineplus, 24 March 2009 .