The thyroid gland, with its distinctive “butterfly” shape, is located above the trachea in the neck.
This gland secrets thyroxine and calcitonin hormons.
Embedded in the thyroid gland are four small parathyroid glands. They produce a hormone called parathyroid hormone (PTH).
Function of thyroxine
Each thyroxine molecule contains four atoms of iodine.
Receptors for thyroxine are found on most cells in the body.
Thyroxine increases basal metabolic rate and oxygen consumption, especially in the heart, skeletal muscle, liver, and kidney.
The increased oxygen demand is due to the stimulation of sodium potassium pump activity in the cell membranes of target cells.
The additional oxygen is consumed in the production of ATP that is required to drive the sodium potassium pump. Heat is given off as a byproduct of this process.
Thyroxine secretion is governed by the anterior lobe of the pituitary gland, which produces thyroid-stimulating hormone (TSH). The two hormones, thyroxine and TSH, interact to adjust the levels of thyroxine in response to the body’s constantly changing needs.
Caffeine in some beverages we consume reduces glucose metabolism in the cells of the body by inhibiting TSH production, which in turn suppresses thyroxine secretion.
Thyroid hormones are carried through the blood by carrier proteins, as are steroid hormones.
Thyroid hormones (circulating in the blood) then feed back to the pituitary gland, where they suppress the secretion of TSH.
In the thyroid gland, TSH stimulates an increase in iodine uptake from the blood, and the synthesis and secretion of thyroxine hormone.
About 30 percent of the iodine in blood is consumed by the thyroid gland to be used in the synthesis of thyroxine.
An excess of thyroxine production is referred to as hyperthyroidism, also known as Grave’s disease.
Grave’s disease is an autoimmune disorder in which antibodies attach to thyroid-stimulating hormone receptors on thyroid cells. This attachment puts receptors in a “perpetually on” mode that stimulates cell division and production of thyroid hormone.
Symptoms of an overactive thyroid
The excessive hormone production causes enlargement of the thyroid, muscle weakness, increased metabolic rate, excessive heat production, and sweating and warm skin due to dilation of blood vessels in the skin (vasodilation).
A deficiency in thyroxine production is referred to as hypothyroidism, or myxedema.
A decrease in thyroxine output can be caused by an iodine deficiency. Decreased thyroxine levels disrupt the negative feedback loop to the pituitary, resulting in continued production of TSH. TSH continues to stimulate cell division in thyroid tissue.
Symptoms of an underactive thyroid
The symptoms of hypothyroidism are like a mirror-image of hyperthyroidism.
Typically, a hypothyroid condition results in reduced basal metabolic rate (which decreases heat production), reduced tolerance of cold temperatures, decreased heart rate and output, and weight gain despite decreased appetite.
Hypothyroidism is also characterized by decreased mental capacity, general weakness and fatigue, and poor physical development.
Calcitonin and Parathyroid Hormone
Calcium levels in the blood are regulated by calcitonin, a hormone which is produced by the thyroid gland, and parathyroid hormone (PTH), which is made by the parathyroid glands.
Calcitonin and parathyroid have opposite effects on blood calcium levels.
High levels of calcium, obtained from dietary sources, stimulate an increase in calcitonin secretion, which then increases the rate at which calcium in blood is deposited into bone tissue of the skeletal system.
Calcitonin acts by increasing the rate of calcium excretion in the kidney.
A decrease in blood calcium prompts the parathyroids to produce more parathyroid hormone.
Increased PTH stimulates bone tissue to release calcium into the blood and increases the rate of re-absorption of calcium from the kidneys and the duodenum of the digestive system.
Parathyroid hormone also promotes vitamin D synthesis in the proximal tubule of the kidney.
The primary role of vitamin D is to maintain blood calcium levels.
Vitamin D increases the release of calcium into the blood from bone tissue.
In the kidney, it increases the retension of calcium.
In the small intestine, vitamin D increases the rate of calcium absorption by stimulating the growth of cells lining the intestine and by stimulating the synthesis of cellular proteins involved in calcium transport.