Anatomy Physiology

The thyroid gland is an endocrine gland that is located on the top of the respiratory tract, anterior to the trachea. The thyroid is composed of follicles that secrete three types of hormones.

Two main types of cells exist in each follicle. These are first the follicular cells and second the parafollicular cells. The follicular cells secrete the two hormones: thyroxine and tri-iodo thyronine. While the parafollicular cells secrete the hormone calcitonine which is important in calcium metabolism in bone tissue.

Increased levels of calcitonin occurs after hypercalcemia or high concentration of calcium in the blood. Calcitonine renders deposition of calcium on bone tissue and thus restoring calcium concentration to its normal level. Calcitonine functions in the opposite way to parathyroid hormone which functions by stimulating the release of calcium from bone tissue to the blood. Calcitonine release is stimulated by excess calcium ions in the blood, while its secretion is inhibited by hypocalcemia or lower levels of calcium in the blood than normal.

Thyroxine which is a hormone that is secreted by the follicular cells of the thyroid gland is important for the metabolism of the body tissues in general. It is converted to the other hormone that is secreted by the follicular cells, which is tri-iodo thyronine. Iodine is an important element for the synthesis of thyroxine and tri-iodo-thyronine. Its deficiency can cause hypothyroidism or low level of the secretion of the thyroid hormone. It can also cause enlargement of the thyroid gland or hyperthyroidism (goiter).

Thyroxine is a lipid soluble hormone. Therefore it is able to penetrate the phospholipid bilayer of the cellular membrane. It makes its effect by binding to nuclear receptors inside the cell where it speeds up the transcription of genes which code for the translation of important enzymes in the metabolic pathway.

Thyroid hormone enhances the cellular metabolism in general. Its secretion is controlled from the pituitary gland by the hormone TSH or thyroid stimulating hormone, which is in turn regulated by a positive feedback from a hormone secreted from the hypothalamus and which is called thyrotropin releasing hormone.

Hypothyroidism is a condition in which the thyroid gland secretes only little thyroid hormone. It is mostly an autoimmune disease in which the body secretes antibodies against the thyroid tissue and thus leading to its destruction. Hypothyroidism can also be triggered by iodine deficiency in the diet. The main form of hypothyroidism is hashimoto thyroiditis.

The other condition that the thyroid gland can have is hyperthyroidism in which it is also an autoimmune disease in which the thyroid secretes a thyroid stimulating thyroglobulin that induces the synthesis and secretion of the thyroid hormones. This condition is also called thyrotoxicosis due to the toxic levels of thyroid hormones that are secreted. Another name is graves disease which has a clinical sign of exophthalmous or protruding eyes.

The treatment for hyperthyroidism is either surgical to remove part of the thyroid gland or by the administration of radioactive iodine in the form of tablets to kill part of the thyroid tissue.

Surgery of the thyroid gland can often result in the inability of the patient to speak. This is due to hurting the laryngeal nerve or injuring it during the surgery. The laryngeal nerve is located on the thyroid gland and can often be injured in thyroid surgery.