|
The Thyroid gland is one of the largest endocrine glands in the body. It is positioned on the neck just below the
Larynx and has two lobes with one on either side of the trachea. It is involved in the production of the hormones T3
(triiodothyronine) and T4 (thyroxine). These hormones increase the metabolic activity of the body�s cells. The
thyroid also produces and releases the hormone calcitonin (thyrocalcitonin) which contributes to the regulation of
blood calcium levels. Thyrocalcitonin or calcitonin decreases the concentration of calcium in the blood. Most of the
calcium removed from the blood is stored in the bones.
The thyroid hormone consists of two components, thyroxine and iodine. This hormone increases the metabolism of
most body cells. A deficiency of iodine in the diet leads to the enlargement of the thyroid gland, known as a simple
goiter. Hypothyroidism during early development leads to cretinism. In adults, it produces myxedema, characterized
by obesity and lethargy. Hyperthyroidism leads to a condition known as exophthalmic goiter, characterized by
weight loss as well as hyperactive and irritable behavior.
The thyroid gland is a two-lobed gland that manifests a remarkably powerful active transport mechanism for
up-taking iodide ions from the blood. As blood flows through the gland, iodide is converted to an active form of
iodine. This iodine combines with an amino acid called tyrosine. Two molecules of iodinated tyrosine then combine
to form thryroxine. Following its formation, the thyroxine becomes bound to a polysaccharide-protein material
called thyroglobulin. The normal thyroid gland may store several weeks supply of thyroxine in this bound form. An
enzymatic splitting of the thyroxine from the thyroglobulin occurs when a specific hormone is released into the
blood. This hormone, produced by the pituitary gland, is known as thyroid-stimulating hormone (TSH). TSH
stimulates certain major rate-limiting steps in thyroxine secretion, and thereby alters its rate of release. A variety of
bodily defects, either dietary, hereditary, or disease induced, may decrease the amount of thyroxine released into the
blood. The most popular of these defects is one that results from dietary iodine deficiency. The thyroid gland
enlarges, in the continued presence of TSH from the pituitary, to form a goiter. This is a futile attempt to synthesize
thyroid hormones, for iodine levels that are too low. Normally, thyroid hormones act via a negative feedback loop on
the pituitary to decrease stimulation of the thyroid. In goiter, the feedback loop cannot be in operation - hence
continual stimulation of the thyroid and the inevitable protuberance on the neck. Formerly, the principal source of
iodine came from seafood. As a result, goiter was prevalent amongst inland areas far removed from the sea. Today,
the incidence of goiter has been drastically reduced by adding iodine to table salt.
Thyroxine serves to stimulate oxidative metabolism in cells; it increases the oxygen consumption and heat
production of most body tissues, a notable exception being the brain. Thyroxine is also necessary for normal growth.
The most likely explanation being that thyroxine promotes the effects of growth hormone on protein synthesis. The
absence of thyroxine significantly reduces the ability of growth hormone to stimulate amino acid uptake and RNA
synthesis. Thyroxine also plays a crucial role in the closely related area of organ development, particularly that of the
central nervous system.
If there is an insufficient amount of thyroxine, a condition referred to as hypothyroidism results. Symptoms of
hypothyroidism stem from the fact that there is a reduction in the rate of oxidative energy-releasing reactions within
the body cells. Usually the patient shows puffy skin, sluggishness, and lowered vitality. Other symptoms of
hypothyroidism include weight gain, decreased libido, inability to tolerate cold, muscle pain and spasm, insomnia
and brittle nails. Hypothyroidism in children, a condition known as cretinism, can result in mental retardation,
dwarfism, and permanent sexual immaturity. Sometimes the thyroid gland produces too much thyroxine, a condition
known as hyperthyroidism. This condition produces symptoms such as an abnormally high body temperature,
profuse sweating, high blood pressure, loss of weight, irritability, and muscular pain and weakness. It also causes the
characteristic symptom of the eyeballs protruding from the skull called exopthalmia. This is surprising because it is
not a symptom usually related to a fast metabolism. Hyperthyroidism has been treated by partial removal or by
partial radiation destruction of the gland. More recently, several drugs that inhibit thyroid activity have been
discovered, and their use is replacing the former surgical procedures. Unfortunately thyroid conditions require
lifetime treatment and because of the body's need for a sensitive balance of thyroid hormone both supplementing and
suppressing thyroid function can take months or even years to regulate.
T3 and T4 Function within the body
The Production of T3 and T4 are regulated by thyroid stimulating hormone (TSH), released by the pituitary gland.
TSH Production is increased when T3 and T4 levels are too low. The thyroid hormones are released throughout the
body to direct the body's metabolism. They stimulate all cells within the body to work at a better metabolic rate.
Without these hormones the body's cells would not be able to regulate the speed at which they performed chemical
actions. Their release will be increased under certain situations such as cold temperatures when a higher metabolism
is needed to generate heat. When children are born with thyroid hormone deficiency they have problems with
physical growth and developmental problems. Brain development can also be severely impaired
The significance of iodine
Thyroid hormone cannot be produced without an abundant source of iodine. The iodine concentration within the
body, although significant, can be as little as 1/25th the concentration within the thyroid itself. When the thyroid is
low on iodine the body will try harder to produce T3 and T4 which will often result in a swelling of the thyroid
gland, resulting in a goiter.
Extrathyroidal iodine
Iodine accounts for 65% of the molecular weight of T4 and 59% of the T3. 15�20 mg of iodine is concentrated in
thyroid tissue and hormones, but 70% of the body's iodine is distributed in other tissues, including mammary glands,
eyes, gastric mucosa, the cervix, and salivary glands. In the cells of these tissues iodide enters directly by
sodium-iodide symporter (NIS). Its role in mammary tissue is related to fetal and neonatal development, but its role
in the other tissues is unknown. It has been shown to act as an antioxidant in these tissues.
The US Food and Nutrition Board and Institute of Medicine recommended daily allowance of iodine ranges from
150 micrograms /day for adult humans to 290 micrograms /day for lactating mothers. However, the thyroid gland
needs no more than 70 micrograms /day to synthesize the requisite daily amounts of T4 and T3. These higher
recommended daily allowance levels of iodine seem necessary for optimal function of a number of body systems,
including lactating breast, gastric mucosa, salivary glands, oral mucosa, thymus, epidermis, choroid plexus, etc.
Moreover, iodine can add to double bonds of docosahexaenoic acid and arachidonic acid of cellular membranes,
making them less reactive to free oxygen radicals.
Calcitonin
Calcitonin is a 32 amino acid polypeptide hormone. It is an additional hormone produced by the thyroid, and
contributes to the regulation of blood calcium levels. Thyroid cells produce calcitonin in response to high calcium
levels in the blood. This hormone will stimulate movement of calcium into the bone structure. It can also be used
therapeutically for the treatment of hypercalcemia or osteoporosis. Without this hormone calcium will stay within
the blood instead of moving into bones to keep them strong and growing. Its importance in humans has not been as
well established as its importance in other animals.
|