Index to this Health
Guide
Thyroid
Hormones
Iodine
Goitre
Hypothalamic - Pituitary - Thyroid Axis
Thyroid Disorders
Graves' Disease
Hashimoto's Thyroiditis
Thyroid Nodules |
Health
Guides on Thyroid Disease
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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 or lobes. 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 "shield".
Thyroid
Hormones
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.
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Iodine
Iodine 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. 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. In Canada and
the U.S., most of the salt is iodized, thus the iodine intake is
more than adequate. Taking excess amounts of iodine in foods such
as kelp will aggravate autoimmune thyroid disease.
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Goitre
Enlargement 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.
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Hypothalamic
- Pituitary - Thyroid Axis
The thyroid gland is influenced by
hormones produced by two other organs:
- The pituitary gland, located at
the base of the brain, produces thyroid stimulating hormone (TSH)
- 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 and the
pituitary. 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 axis."
The way 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, insufficient thyroid hormone is produced by the thyroid;
hypothalamic TRH causes TSH to be released from the pituitary
in large amounts. The pituitary also resonds directly to the lack
of thyroid hormone in the blood and TSH is increased. 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 cells.
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 level.
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 production.
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Thyroid
Disorders
The main causes of thyroid disease
are:
- too much thyroid hormone production
or hyperthyroidism.
- 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. Graves' disease affects about 1 in 100 of the population,
whereas hashimoto's thyroiditis is even more common.
Other less common causes of thyroid
disease include nodule, thyroid cancer, subacute thyroiditis and
primary hypothyroidism.Nodules, mostly benign are very common.
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Graves
Disease
Graves' disease (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.
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Hashimoto's
Thyroiditis
In Hashimoto's 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. |
Thyroid
Nodules
Sometimes, thyroid 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 mutations
of follicular cells.
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printed version of this Health Guide is available to health care
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Production of
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the official policy of Health Canada. |
