"A delicate Feedback Mechanism"
thyroid gland is a small gland, normally weighing less than one ounce, located
in the front of the neck. It is made up of two halves, called lobes, that lie
along the windpipe (trachea) and are joined together by a narrow band of thyroid
tissue, known as the isthmus.
The thyroid is situated just below your "Adams apple"
or larynx. During development (inside the womb) the thyroid gland originates in
the back of the tongue, but it normally migrates to the front of the neck before
birth. Sometimes it fails to migrate properly and is located high in the neck or
even in the back of the tongue (lingual thyroid) This is very rare. At other
times it may migrate too far and ends up in the chest (this is also rare).
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.
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).
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.