Hyperthyroidism (or "overactive thyroid gland") is the clinical syndrome caused by an excess of circulating free thyroxine (T4) or free triiodothyronine (T3), or both.
Major clinical features in humans are weight loss (often accompanied by a ravenous appetite), intolerance to heat, 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), shortness of breath (dyspnea), loss of libido, nausea, vomiting, and diarrhea.
In the elderly, these classical symptoms may not be present and they may present only with fatigue and weight loss leading to apathetic hyperthyroidism.
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