Hypothyroidism: Given the patient's low BMI, bradycardia (heart rate 46), and feeling cold, hypothyroidism is a plausible diagnosis. Hypothyroidism can cause decreased metabolism, leading to weight loss, cold intolerance, and bradycardia.

Anorexia Nervosa: With a BMI of 17.3, this patient is underweight. Anorexia nervosa could explain the low BMI, bradycardia, and possibly the feeling of being cold due to poor nutrition and decreased body fat.
Dehydration or Electrolyte Imbalance: Although less directly linked to all symptoms, dehydration or electrolyte imbalances can cause bradycardia and potentially make someone feel cold due to decreased blood volume or altered nerve conduction.
Anemia: Could potentially cause feelings of coldness due to decreased oxygen delivery to tissues, though it's less directly linked to the heart rate and blood pressure findings.

Myocardial Ischemia or Cardiac Issues: Although less common in a 20-year-old, cardiac issues such as myocardial ischemia or cardiomyopathy could cause bradycardia and potentially lead to feelings of coldness due to decreased cardiac output. These conditions are critical and require immediate attention.
Adrenal Insufficiency: This condition can cause hypotension, bradycardia, and potentially feelings of coldness. It's a life-threatening condition if not promptly treated.
Sepsis: While the blood pressure is currently within a relatively normal range, sepsis can present with bradycardia in the early stages, especially in younger patients, and can rapidly progress to a life-threatening condition.

Pituitary Adenoma: A tumor in the pituitary gland could lead to hypothyroidism (if it affects TSH production) or other hormonal imbalances that might explain some of the symptoms.
Mitochondrial Myopathies: Rare genetic disorders affecting muscle and nerve function, which could potentially cause a combination of these symptoms, though this would be quite unusual.
Autonomic Dysfunction: Conditions affecting the autonomic nervous system could potentially cause bradycardia and altered temperature regulation, though these are less common and would likely be accompanied by other symptoms.