Hyperthyroidism is characterised by high levels of serum thyroxine (T4), high levels of serum triiodothyronine (T3), or both, and low levels of thyroid-stimulating hormone (TSH, also known as thyrotropin). Subclinical hyperthyroidism is characterised by decreased levels of TSH (<0.1 mU/L) but with levels of T4 and T3 within the normal range (total T4: 60–140 nanomol/L; total T3: 1.0–2.5 nanomol/L, depending on assay type). The terms hyperthyroidism and thyrotoxicosis are often used synonymously; however, they refer to slightly different conditions. Hyperthyroidism refers to overactivity of the thyroid gland leading to excessive production of thyroid hormones. Thyrotoxicosis refers to the clinical effects of unbound thyroid hormones, whether or not the thyroid gland is the primary source. Secondary hyperthyroidism due to pituitary adenomas, thyroiditis, iodine-induced hyperthyroiditis, and treatment of children and pregnant or lactating women are not covered in this review. Hyperthyroidism can be caused by Graves' disease (diffusely enlarged thyroid gland on palpation, ophthalmopathy, and dermopathy), toxic multinodular goitre (thyrotoxicosis and increased radioiodine uptake with multinodular goitre on palpation), or toxic adenoma (benign hyperfunctioning thyroid neoplasm presenting as a solitary thyroid nodule). We have not included treatment of Graves' ophthalmopathy in this review, although we do report on worsening of Graves' ophthalmopathy with radioiodine. We have also not included euthyroid sick syndrome (a condition seen in people with, for example, pneumonia, MI, cancer, and depression — it is characterised by low levels of TSH and T3). Diagnosis: The diagnosis of hyperthyroidism is established by a raised serum total or free T4 or T3 hormone levels, reduced TSH level, and high radioiodine uptake in the thyroid gland along with features of thyrotoxicosis. The usual symptoms are irritability, heat intolerance and excessive sweating, palpitations, weight loss with increased appetite, increased bowel frequency, and oligomenorrhoea. People with hyperthyroidism also often have tachycardia, fine tremors, warm and moist skin, muscle weakness, and eyelid retraction or lag.
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