Final answer:
Sinus bradycardia can be caused by calcium channel and beta-blocker medications, athletic conditioning, and hypothermia, but not typically by hyperthyroidism, which generally results in an increased heart rate.
Step-by-step explanation:
Sinus bradycardia can be a symptom associated with several conditions. It can be caused as a side effect of certain medications and also as a result of certain physiological states. Specifically, sinus bradycardia may develop due to the use of calcium channel blocker and beta-blocker medication, through athletic conditioning, in response to hypothermia, and is counterintuitive to hyperthyroidism, which usually increases heart rate.
Beta-blockers, such as metoprolol or propranolol, slow heart rate by blocking the beta-1 receptors that are associated with cardioacceleration. Calcium channel blockers also reduce the strength of heart contractions and thus can reduce heart rate. Athletic conditioning can lead to a lower resting heart rate, as the heart becomes more efficient at pumping blood. However, hypothermia is a factor that decreases heart rate by altering the metabolic and electrophysiological processes of the heart. Conversely, hyperthyroidism typically causes tachycardia, not bradycardia, due to the over-stimulation of the heart and metabolic rate.