Which endocrine abnormality is most likely to contribute to palpitations and an irregularly irregular rhythm in a patient with an elevated heart rate?

Hyperthyroidism is the most likely endocrine abnormality to contribute to palpitations and an irregularly irregular rhythm in a patient with an elevated heart rate. This condition occurs when the thyroid gland produces excessive amounts of thyroid hormones, leading to an increase in metabolic processes throughout the body, including heightened heart rate and cardiac excitability.

The increased levels of thyroid hormone can lead to various cardiovascular symptoms, one of which is tachycardia (an elevated heart rate). Furthermore, these hormones can cause atrial fibrillation, which is characterized by an irregularly irregular heartbeat. This arrhythmia occurs due to the effects of excessive thyroid hormone on the electrical conduction system of the heart, contributing to the patient's complaints of palpitations.

Other endocrine disorders, while they can affect cardiovascular health, do not typically present with the specific combination of symptoms described. Addison's disease, for example, results in low cortisol levels, which can lead to decreased blood pressure and altered cardiovascular response. Cushing's syndrome is characterized by excessive cortisol but does not commonly produce an irregularly irregular rhythm like hyperthyroidism does. Hyperparathyroidism mainly affects calcium metabolism and can influence cardiac function but is less likely to present with the symptoms as defining as those caused by hyperthyroidism.