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The thyroid (/ˈθaɪrɔɪd/; from the Greek word for door) is a small endocrine (hormone-producing) organ located in the front of the neck just below the larynx ("Adam's apple"). The thyroid is responsible for regulating many processes in the human body.


The thyroid processes iodine into two hormones: thyroxine (T4) and triiodythyromine (T3). These two hormones control metabolism. It also produces calcitonin, which is necessary for calcium absorption. The thyroid is controlled by the pituitary gland.

The pituitary releases Thyroid stimulation hormone (TSH) in response to T3 and T4 levels circulation in the blood. If there is an excess of T4, TSH production is diminished; if there isn't enough T4, TSH production is increased.


Iodine, which is found in many foods, is the essential nutrient in thyroid function. If one does not consume enough iodine, the thyroid becomes enlarged, which leads to goiter. This is the reason that we see iodized salt—to prevent growth deficiencies related to a lack of iodine in the diet.


Hypothyroidism is a lack of T3/T4 in the blood stream; hyperthyroid is an excess. The chief diagnostic tool for these illnesses is that TSH level. The TSH level is inversely proportionate to the level of thyroid function (i.e. a high TSH level means an hypoactive thyroid). Signs and symptoms include:


  • Fatigue
  • Intolerance to cold
  • Depression
  • Constipation
  • Paleness
  • Weight gain and/or water retention
  • Bradycardia (slow heart rate)

Treatment for hypothyroid involves thyroid-hormone replacement therapy (thyroxine).

Hyperthyroid (Graves disease)

  • Anxiety or feeling "hyper"
  • Intolerance to head
  • Hair loss
  • Polydipsia (uncontrollable thirst)
  • Tachycardia (fast hear rate) or dysrhythmia

Treatment for hyperthyroid is more complicated. Some cases require surgical intervention to remove part or all of the thyroid. In most cases, alleviation of symptoms is achieved by irradiated iodine. The irradiated iodine destroys a part of the overactive gland, returning the T3/T4 levels to normal.

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