Monday, May 27, 2013

Endocrine System #3: Thyroid Gland

Located at the base of the neck, the thyroid gland is one of the biggest endocrine glands in the body.

It's main function is to produce thyroid hormones, which are used for regulating the body's metabolism.

Thyroid hormones
  • Derived from tyrosine
  • Diffuse into the cell and bind to receptors in the nucleus. 
  • Increase glucose metabolism, protein synthesis, and oxygen consumption
  • Regulate growth and tissue differentiation (bone formation, digestive system, nerve cells, muscle tone, and reproductive cells)
  • Thyroxine (T4): four iodines; the majority of the hormones made in the thyroid gland, but main purpose is to be converted to T3.
  • Triiodothyronine (T3): three iodines; much more potent than T4.

Thyroid hormones are secreted when metabolic rate is low. The low metabolic rate is sensed by the hypothalamus, which secretes the thyrotropin releasing hormone (TRH). The TRH then triggers the anterior pituitary gland to secrete thyroid stimulating hormone (TSH), which would trigger the thyroid gland to produce T3 and T4 hormones.

In general, hyperthyroidism has the following symptoms (mostly due to increased metabolism): weight loss, increased appetite, increased bowel movement, increased body heat, light or absence of menstrual activities, fatigue, and irritability. 

Hyperthyroidism can be caused by iodine deficiency. Due to the low iodine levels, there are also low levels of T3 and T4, so the pituitary gland stimulates more and more TSH. As a result, there is overactivity in the thyroid gland. This in turn causes the thyroid gland to form a goiter

Hyperthyroidism can also be caused by Grave's disease, which is an autoimmune disease. The patient's body produces an auto-antibody called thyroid stimulating immunoglobin (TSI). These antibodies bind to the TSH receptors, and continuously enable them. As a result, abnormally high levels of T3 and T4 are synthesized. Patients with Grave's disease also have protruding eyes and eye irritation.

Other causes of hyperthyroidism include thyroiditis, pituitary tumour, excess iodine, and overactive thyroid glands.

Hyperthyroidism can be treated by:
  • drug therapy (drugs that block the thyroid gland, prevent entry of iodine into the thyroid gland, or block the thyroid hormones in the blood stream)
  • radioactive iodine therapy (radioactive iodine 131)
  • surgery: partial or complete removal of the thyroid gland

The opposite of hyperthyroidism, hypothyroidism is characterized by a shortage of thyroid hormones. It results in weight gain, fatigue, and decreased heart rate. 

Hypothyroidism is mainly caused by Hashimoto's thyroiditis, in which the immune system attacks the thyroid gland and causes the its inflammation. It can be treated by thyroid hormone supplements.