Hyperthyroidism (thyrotoxicosis or "fast thyroid gland") is the clinical syndrome caused by an excess of circulating free thyroxine (T4) and 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, and subacute thyroiditis.

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, infertility, loss of libido, nausea, vomiting, and diarrhea.

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 well been 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.

Diagnosis

A diagnosis is made through a blood test, by measuring the level of T4 in the blood. High T4 levels are considered indicative of hyperthyroidism. If the index of suspicion is low, many doctors prefer to measure thyroid-stimulating hormone (TSH). If TSH is suppressed, there may be uncontrolled production of T4, while a normal TSH generally rules out thyroid disease. Measuring specific antibodies, such as anti-TSH-receptor antibodies in Graves' disease, may contribute to the diagnosis. Additionally, scintigraphy may be required.

Treatment

The major and generally accepted modalities for treatment of hyperthyroidism in humans are:

• surgery
• radioiodine treatment
• thyrostatics - drugs that inhibit the production of thyroid hormones, such as methimazole (Tapazole®)

If too high a dose is used in pharmacological treatment, patients can develop symptoms of 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.

In animals

In 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, diarrhea, increased water consumption and increased urine production.

Surgery is not normally an option in feline hyperthyroidism. Radioiodine treatment or methimazole may be used to control symptoms.