Hyperthyroidism 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 reason 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, 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.
A diagnosis is made through a blood test depressed levels of TSH and increased levels of T3 and T4.
A normal TSH level excludes both hyper / hypothyroidism. However, because small changes T3 and T4 levels can be detected by TSH assay. T3 and T4 levels must be used for confirmation. The test used commonly is total T4 level which correlates well with free T4 level in the absence of any disturbance in TBG levels. Because alterations in TBG levels alters the levels of total T4 without affecting free T4 levels; a reasonable estimate of hormone binding capacity is T3 resin uptake which varies inversely with hormone binding capacity and should be used in conjunction with total T4. The T4 index is a rough estimate of free T4 and is the product of total T4 with T3 resin uptake.
As to other autoimmune disorders related with thyrotoxicosis, an association between thyroid disease and myasthenia gravis has well been recognized. 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.
The major and generally accepted modalities for treatment of hyperthyroidism are:
- Radioiodine treatment
- Drugs that inhibit the production of thyroid hormones, such as methimazole (also known by the brand name 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.