Hyperthyroidism (cont.)
What causes hyperthyroidism?
Some common
causes of hyperthyroidism include:
- Graves' Disease
- Functioning adenoma ("hot nodule") and
Toxic Multinodular Goiter (TMNG)
- Excessive intake of thyroid hormones
- Abnormal secretion of TSH
- Thyroiditis (inflammation of the
thyroid gland)
- Excessive iodine intake
Graves' Disease
Graves'
disease, which is caused by a generalized overactivity of the thyroid gland, is
the most common cause of hyperthyroidism. In this condition, the thyroid gland
usually is renegade, which means it has lost the ability to respond to the
normal control by the pituitary gland via TSH. Graves'
disease is hereditary and is up to five times more common among women than men. Graves'
disease is thought to be an autoimmune disease, and antibodies that are
characteristic of the illness may be found in the blood. These antibodies
include thyroid stimulating immunoglobulin (TSI antibodies),
thyroid peroxidase antibodies (TPO), and TSH receptor antibodies. The
triggers for Grave's disease include:
- stress,
- smoking,
- radiation to the neck,
- medications, and
- infectious organisms such as viruses.
Graves' disease can be diagnosed by a standard, nuclear medicine
thyroid scan which shows diffusely increased uptake of a
radioactively-labeled iodine. In addition, a blood test may reveal elevated TSI levels.
Grave's disease may be associated with eye disease
(Graves' ophthalmopathy) and skin lesions (dermopathy
). Ophthalmopathy can occur before, after, or at the same time as
the hyperthyroidism. Early on, it may cause sensitivity to light and a feeling of
"sand in the eyes." The eyes may protrude and double vision can occur. The degree
of ophthalmopathy is worsened in those who smoke. The course of the eye disease
is often independent of the thyroid disease, and steroid therapy may be necessary
to control the inflammation that causes the ophthalmopathy. In addition, surgical intervention may be required.
The skin condition (dermopathy) is rare and causes a painless, red , lumpy skin
rash that appears on the front of the
legs.
Functioning Adenoma and Toxic
Multinodular Goiter
The thyroid gland (like many other areas of the body) becomes lumpier as we
get older. In the majority of cases, these lumps do not produce thyroid hormones
and require no treatment. Occasionally, a nodule may become "autonomous," which means that it does not respond to pituitary
regulation via TSH and produces thyroid hormones independently. This becomes more likely if the
nodule is larger that 3 cm. When there is a single nodule that is
independently producing thyroid hormones, it is called a functioning nodule. If there is more
than one functioning nodule, the term toxic, multinodular goiter is used. Functioning
nodules may be readily detected with a thyroid scan.
Excessive intake of thyroid hormones
Taking too much
thyroid hormone medication
is actually quite common. Excessive doses of
thyroid hormones frequently go undetected due to the lack of follow-up of patients
taking their thyroid medicine. Other persons may be abusing the drug in an
attempt to achieve other goals such as weight loss. These patients can be identified
by having a low uptake of radioactively-labelled iodine (radioiodine) on a thyroid scan.
Abnormal secretion of TSH
A tumor in the pituitary gland may produce an abnormally high secretion of TSH (the
thyroid stimulating hormone). This leads to excessive signaling to the thyroid gland to
produce thyroid hormones. This condition is very rare and can be associated with
other abnormalities of the pituitary gland. To identify this disorder, an
endocrinologist performs elaborate tests to assess the release of TSH.
Thyroiditis (inflammation of the thyroid)
Inflammation of
the thyroid gland may occur after a viral illness (subacute
thyroiditis). This
condition is association with a fever and a sore throat that is
often painful on swallowing. The thyroid
gland is also tender to touch. There may be generalized neck aches and
pains. Inflammation of the gland with an accumulation of white blood cells known as
lymphocytes (lymphocytic thyroiditis) may also occur. In both of these
conditions, the inflammation leaves the thyroid gland "leaky," so that the
amount of thyroid hormone entering the blood is increased. Lymphocytic
thyroiditis is most common after a pregnancy and
can actually occur in up to 8% of women after delivery. In these cases, the
hyperthyroid phase can last from 4 to 12 weeks and is often followed by a
hypothyroid (low thyroid output) phase that can last for up to 6 months. The
majority of affected women return to a state of normal thyroid function.
Thyroiditis can be diagnosed by a thyroid scan.
Excessive iodine intake
The thyroid gland uses iodine to
make thyroid hormones. An excess of iodine may cause hyperthyroidism.
Iodine-induced hyperthyroidism is usually seen in patients who already have an
underlying abnormal thyroid gland. Certain medications, such as amiodarone
(Cordarone), which is used in the treatment of heart problems, contain a large amount of iodine and may be associated with
thyroid function abnormalities.
Next: What are the symptoms of hyperthyroidism? »
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