Dr. MJ Bazos MD,
The thyroid gland is located in the front of
the neck attached to the lower part of the voicebox (or larynx) and to the
upper part of the windpipe (or trachea). It has two sides orlobes. These lobes
are connected by a narrow neck (or isthmus). Each lobe is about 4 cm long and
1 to 2 cm wide. The name "thyroid" comes from the Greek word which means
The thyroid gland produces
thyroid hormones. These are peptides containing iodine. The two most important
hormones are tetraiodothyronine (thyroxine or T4) and triiodothyronine (T3).
These hormones are essential for life and have many effects on body
metabolism, growth, and development.
plays an important role in the function of the thyroid gland. It is the chief
component of thyroid hormones, and is essential for their production. Iodine is
obtained from the water we drink and the food we eat. The taking of excess
amounts of iodine, such as kelp, will aggravate autoimmune thyroid disease and
make the antibodies higher. In areas of the world where there is an iodine
deficiency, iodine must be added to the salt or bread. The Great Lakes area of
Canada and the U.S., the Swiss Alps and Tasmania are such areas.
of the thyroid gland is called goitre. Goitre does not always indicate a
disease, since thyroid enlargement can also be caused by physiological
conditions such as puberty and pregnancy.
Hypothalamic - Pituitary -
The thyroid gland is
influenced by hormones produced by two other organs:
1. The pituitary gland, located at the
base of the skull produces thyroid stimulating hormone (TSH)
2. The hypothalamus, a small part of the
brain above the pituitary, produces thyrotropin releasing hormone (TRH).
Low levels of thyroid hormones in the
blood are detected by the hypothalamus. TRH is released, stimulating the
pituitary to release TSH. Increased levels of TSH, in turn, stimulate the
thyroid to produce more thyroid hormone, thereby returning the level of thyroid
hormone in the blood back to normal.
The three glands and the hormones they
produce make up the "Hypothalamic - Pituitary - Thyroid
The ways a goitre forms in those
geographic areas of the world which have a deficiency of iodine is a good
example of how the axis functions. Normally, TSH increases the uptake of iodine
by the thyroid gland and increases production of thyroid hormone. If there is
little iodine available in our diet, hypothalamic TRH causes TSH to be
released from the pituitary in large amounts. This enables the thyroid to
capture most of the iodine presented to it from food and water. But, TSH has a
second action - it causes growth of thyroid
The gland grows and becomes very
large under the influence of this high level of TSH secretion. Therefore, most
people who live in iodine deficient areas have goitre, thus allowing them to
produce enough thyroid hormone for normal body function. Once thyroid hormone
levels are restored, TSH secretion stabilizes at a high
In healthy individuals and in
those with goitre, the hypothalamic - pituitary - thyroid axis maintains
thyroid hormone production at a finely controlled level and enables the thyroid
to respond to situations requiring more or less thyroid hormone
The main causes of
thyroid disease are:
1. Too much
thyroid hormone production or hyperthyroidism.
2. Too little thyroid hormone
production or hypothyroidism.
The state of normal thyroid
function is called euthyroidism.
Abnormalities of the thyroid gland are
common and affect one in twenty (1 in 20) of the Canadian population. All
thyroid disorders are much more common in women than in men. Because of the
widespread use of iodized salt and bread, lack of iodine is no longer a cause
of thyroid disease in Canada as it was some 50 years ago.
"Autoimmune disorders" of the thyroid gland are
common. These autoimmune disorders are caused by abnormal proteins, (called
antibodies), and the white blood cells which act together to stimulate or damage
the thyroid gland. Graves' disease (hyperthyroidism) and Hashimoto's
thyroiditis, are diseases of this type. Each affects about 1 in 100 of the
Other less common causes of
thyroid disease include nodule, thyroid cancer, subacute thyroiditis and
(thyrotoxicosis) is due to a unique antibody called "thyroid stimulating
antibody" which stimulates the thyroid cells to grow larger and to produce
excessive amounts of thyroid hormones. In this disease, the goitre is due not
to TSH but to this unique antibody.
thyroiditis, the goitre is caused by an accumulation of white blood cells and
fluid (inflammation) in the thyroid gland. This leads to destruction of the
thyroid cells and, eventually, thyroid failure (hypothyroidism). As the gland
is destroyed, thyroid hormone production decreases; as a result, TSH increases,
making the goitre even larger.
enlargement is restricted to one part of the gland; the rest of the gland being
normal. The most common cause of this is a cyst or nodule, which may be benign
or malignant. Occasionally there are many nodules. This so called "multinodular
goitre" is probably caused by an abnormality of iodine metabolism.