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Thyroid hormone affects all organ systems and is responsible for increasing metabolic rate, heart rate, and ventricle contractility, as well as muscle and central nervous system excitability. Two major types of thyroid hormones are thyroxine and triiodothyronine. Thyroxine is the major form of thyroid hormone. The ratio of thyroxine to triiodothyronine released in the blood is 20:1. Peripherally, thyroxine is converted to the active triiodothyronine, which is three to four times more potent than thyroxine.

Hyperthyroidism refers to excess circulating hormone resulting only from thyroid gland hyperfunction, whereas thyrotoxicosis refers to excess circulating thyroid hormone originating from any cause (including thyroid hormone overdose).

Thyroid storm is the extreme manifestation of thyrotoxicosis. This is an acute, severe, life-threatening hypermetabolic state of thyrotoxicosis caused either by excessive release of thyroid hormones causing adrenergic hyperactivity or altered peripheral response to thyroid hormone following the presence of one or more precipitants.

The mortality of thyroid storm without treatment is between 80% and 100%, and with treatment, it is between 15% and 50%.

Primary hyperthyroidism is caused by the excess production of thyroid hormones from the thyroid glands. Secondary hyperthyroidism is caused by the excess production of thyroid-releasing hormones or thyroid-stimulating hormones in the hypothalamus and pituitary, respectively (Tables 229–1 and 229–2).

Table 229–1

Causes of Hyperthyroidism: Primary and Secondary Hyperthyroidism

Table 229–2

Other Causes of Hyperthyroidism

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