The thyroid gland is a small butterfly
shaped gland located in the neck just below the Adam's apple and above
the collar bone. It sits in front of the trachea. The word thyroid is
Greek for "shield shaped".
The thyroid gland weighs approximately 20
grams. Each lobe is approximately 4 cm (1.6 inches) from top to bottom.
During certain conditions, such as iodine
deficiency, the thyroid gland enlarges in order to capture more iodine
from the blood to create thyroid hormones. The thyroid is supplied with
blood through 4 major arteries. Under normal conditions these arteries
carry the equivalent of the entire body's supply of blood each hour.
Adjacent to each thyroid lobe is the
laryngeal nerve which supplies the voice box and vocal cords. Because
of this there is a risk of damage during thyroid surgeries.
Four parathyroid glands (near the
thyroid) make parathyroid hormones (PTH). This causes the kidneys to
retain calcium in the blood thus releasing phosphorus into the urine.
PTH also increases the activation of Vit D which enhances the activity
of Vit D & Calcium. Parathyroid glands are sometimes damaged in
surgery or are accidentally removed leaving the patient to suffer from
hypoparathyroidism for the rest of their life. This causes calcium
levels to drop giving symptoms of muscle cramps & spasms,
numbness and if severe, seizures.
Inside the thyroid are cells called
follicular cells. They are arranged in ball shaped groups called
follicles with the center (follicular lumen) containing stored thyroid
hormone called a colloid. All of the thyroid follicles are contained in
a fibrous capsule of the thyroid gland like bunches of grapes. The
adjacent cells called parafollicular cells make additional hormones
like calcitonin. The thyroid follicular cells have receptors (proteins)
in the membrane for receiving hormones that control the thyroid like
TSH (thyroid stimulating hormone - from the pituitary gland). TSH
signals the NIS (sodium iodine symporters or pumps) to pull iodine into
the cells to produce thyroid hormone. TSH also signals the
body to produce thyroglobulin which is used in the thyroid hormone