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Thyrotoxicosis is a disease caused by excessive concentrations of free thyroid hormones (most often T4).

Many of the clinical features of thyrotoxicosis are characterised by signs of sympathetic overactivity such as tremor, tachycardia, and sweating producing weight loss, fatigue and heat intolerance.

The level of catecholamines is normal but the excess thyroid hormones seem to potentiate the action of the catecholamines.

Hyperthyroidism is extremely common, affecting perhaps 2 to 5% of all females at some time in their lives. The vast majority of cases are caused by intrinsic thyroid disease; a pituitary cause is extremely rare.