Hyperthyroidism (or "overactive
thyroid gland") is the clinical syndrome caused by an excess of circulating free
thyroxine (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 thyroiditis (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
weight loss (often accompanied by a ravenous appetite), fatigue, weakness,
hyperactivity, irritability, apathy, depression, polyuria, and sweating.
Additionally, patients may present with a variety of symptoms such as
palpitations and arrhythmias (notably atrial fibrillation), dyspnea, loss of
libido, nausea, vomiting, and 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
tremor, chorea, myopathy, and 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 pseudotumor cerebri, amyotrophic
lateral sclerosis and a Guillain-Barré-like syndrome.
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, scintigraphy is required in order to distinguish true hyperthyroidism from thyroiditis.
The major and generally accepted modalities for treatment of hyperthyroidism
in humans are:
(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.
In Radioiodine (treatment) 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 - hypothyroidism. However, that is usually
easily treated by the administration of levothyroxine, 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
levothyroxine 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 adenoma.
The most common presenting symptoms are: rapid
weight loss, rapid heart rate, vomiting, diarrhoea, increased water consumption
and increased urine production.
Surgery is not normally an option in feline hyperthyroidism. Radioiodine
methimazole may be used to control symptoms.