Hyperthyroidism (or "overactive
is the clinical
syndrome caused by an excess of circulating free
(T4) or free
triiodothyronine (T3), or both.
Major causes in humans are:
- Graves' disease (the most common etiology with 70-80%)
Toxic thyroid adenoma
Toxic multinodular goitre
Other causes of
hyperthyroxinemia (high blood levels of thyroid hormones) are not to be
confused with true hyperthyroidism and include subacute and other forms of
(inflammation). Thyrotoxicosis (symptoms caused by hyperthyroxinemia) can occur
in both hyperthyroidism and thyroiditis. When it causes acutely increased
metabolism, it is sometimes called "thyroid storm".
Signs and symptoms
Major clinical features in humans are
(often accompanied by a ravenous
fatigue, weakness, hyperactivity, irritability,
and sweating. Additionally, patients may present with a variety of symptoms such
infertility, loss of libido,
diarrhea. In the
elderly, these classical symptoms may not be present and they may present only
with fatigue and weight loss leading to apathetic hyperthyroidism
Neurological manifestations are
periodic paralysis. Stroke
of cardioembolic origin due to coexisting
atrial fibrillation may be mentioned as one of the most serious
complications of hyperthyroidism.
As to other autoimmune disorders related with thyrotoxicosis, an association
between thyroid disease and
myasthenia gravis has been well recognised. The thyroid disease, in this
condition, is often an autoimmune one and approximately 5% of patients with
myasthenia gravis also have hyperthyroidism. Myasthenia gravis rarely improves
after thyroid treatment and relation between two entities is yet unknown. Some
very rare neurological manifestations that are reported to be dubiously
associated with thyrotoxicosis are
amyotrophic lateral sclerosis and a
A diagnosis is suspected through blood tests, by measuring the level of TSH
(thyroid stimulating hormone) in the blood. If TSH is low, there is likely to be
increased production of T4 and/or T3. Measuring specific
antibodies, such as
anti-TSH-receptor antibodies in Graves' disease, may contribute to the
diagnosis. In all patients with hyperthyroxinemia,
required in order to distinguish true hyperthyroidism from thyroiditis.
The major and generally accepted modalities for treatment of hyperthyroidism
in humans are:
Surgery (to remove the
whole thyroid or a part of it) is not extensively used because most common forms
of hyperthyroidism are quite effectively treated by the radioactive iodine
method. However, some Graves' disease patients who cannot tolerate medicines for
one reason or another or patients who refuse radioiodine opt for surgical
intervention. The procedure is relatively safe - some surgeons are even treating
partial thyroidectomy on an out-patient basis.
therapy, radioactive iodine is given orally (either by pill or liquid) on a
one-time basis to ablate a hyperactive gland. The iodine given for ablative
treatment is different from the iodine used in a scan. Radioactive iodine is
given after a routine iodine scan, and uptake of the iodine is determined to
confirm hyperthyroidism. The radioactive iodine is picked up by the active cells
in the thyroid and destroys them. Since iodine is only picked up by thyroid
cells, the destruction is local, and there are no widespread side effects with
this therapy. Radioactive iodine ablation has been safely used for over 50
years, and the only major reasons for not using it are pregnancy and
Often, due to the difficulty of picking the correct dose, the treatment
results in an opposite condition -
However, that is usually easily treated by the administration of
which is a pure synthetic form of T4.
Thyrostatics are drugs that inhibit the production of thyroid hormones, such
as methimazole (Tapazole®)
or PTU (propylthiouracil).
If too high a dose is used in pharmacological treatment, patients can develop
hypothyroidism. Hypothyroidism is also a very common result of surgery or
radiation treatment as it is difficult to gauge how much of the thyroid gland
should be removed. Supplementation with
may be required in these cases.
veterinary medicine, hyperthyroidism is one of the most common
endocrine conditions affecting older domesticated
cats. The disease has become
significantly more common since the first reports of feline hyperthyroidism in
the 1970s. In cats, it is
almost always caused by a benign thyroid
The most common presenting symptoms are: rapid
increased water consumption and
increased urine production.
Surgery is not normally an option in feline hyperthyroidism. Radioiodine
methimazole may be used to control symptoms.
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