What symptom would most indicate a patient is undergoing a hyperthyroid crisis?

Tachycardia is a hallmark symptom of a hyperthyroid crisis, also known as thyrotoxic crisis or thyroid storm. In this state, the patient's metabolism is significantly accelerated due to excessively high levels of thyroid hormones. This heightened metabolic state leads to an increased heart rate as the body attempts to cope with the increased demand for oxygen and energy.

Elevated heart rates can often exceed 100 beats per minute and may even reach dangerously high levels in severe cases. This symptom is critical because it can indicate the severity of the condition and the need for immediate medical intervention to prevent cardiovascular complications. The presentation of tachycardia, in conjunction with other symptoms like fever, agitation, and symptoms of heart failure, underscores the urgency of the situation typically seen in a hyperthyroid crisis.

In contrast, symptoms such as hypothermia, lethargy, and decreased systolic blood pressure are not characteristic of hyperthyroid crisis. Hypothermia is more associated with hypothyroidism, while lethargy and decreased blood pressure can indicate various other conditions but are not primary indicators of an overactive thyroid state.