The thyroid gland is the biggest gland in the neck. It is situated in the
anterior (front) neck below the skin and muscle layers.
The thyroid gland takes the shape of a butterfly with the
two wings being represented by the left and right thyroid
lobes which wrap around the trachea. The
sole function of the thyroid is to make thyroid hormone.
This hormone has an effect on nearly all tissues of the
body where it increases cellular activity. The function
of the thyroid therefore is to regulate the body's metabolism.
The function of the thyroid gland is to
take iodine, found in many foods, and convert it into thyroid
hormones: thyroxine (T4) and triiodothyronine (T3). Thyroid
cells are the only cells in the body which can absorb iodine.
These cells combine iodine and the amino acid tyrosine to
make T3 and T4. T3 and T4 are then released into the blood
stream and are transported throughout the body where they
control metabolism (conversion of oxygen and calories to
energy). Every cell in the body depends upon thyroid hormones
for regulation of their metabolism. The normal thyroid gland
produces about 80% T4 and about 20% T3, however, T3 possesses
about four times the hormone "strength" as T4.
The thyroid gland is under the control of
the pituitary gland, a small gland the size of a peanut
at the base of the brain (shown here in orange). When the
level of thyroid hormones (T3 & T4) drops too low, the
pituitary gland produces Thyroid Stimulating Hormone (TSH)
which stimulates the thyroid gland to produce more hormones.
Under the influence of TSH, the thyroid will manufacture
and secrete T3 and T4 thereby raising their blood levels.
The pituitary senses this and responds by decreasing its
TSH production. One can imagine the thyroid gland as a furnace
and the pituitary gland as the thermostat. Thyroid hormones
are like heat. When the heat gets back to the thermostat,
it turns the thermostat off. As the room cools (the thyroid
hormone levels drop), the thermostat turns back on (TSH
increases) and the furnace produces more heat (thyroid hormones).
The pituitary gland itself is regulated
by another gland, known as the hypothalamus(shown in our
picture in light blue). The hypothalamus is part of the
brain and produces TSH Releasing Hormone (TRH) which tells
the pituitary gland to stimulate the thyroid gland (release
TSH). One might imagine the hypothalamus as the person who
regulates the thermostat since it tells the pituitary gland
at what level the thyroid should be set.
Ref: http://www.endocrineweb.com/thyroid.html and http://www.endocrineweb.com/thyfunction.html
