A 46-year-old woman is exhibiting symptoms like weight gain, sensitivity to cold, and a pulse of 50 bpm. What is the single most likely diagnosis?

The symptoms of weight gain, sensitivity to cold, and bradycardia (a pulse of 50 bpm) are highly indicative of hypothyroidism. This condition occurs when the thyroid gland does not produce sufficient amounts of thyroid hormones, leading to a decrease in the body's metabolic rate.

Weight gain in hypothyroidism is a consequence of a slowed metabolism, which can also result in fatigue and an increased sensitivity to cold due to poor thermoregulation. Additionally, bradycardia is a common cardiac manifestation of this condition, as decreased thyroid hormone levels can lead to reduced heart rate by affecting the autonomic nervous system's control over heart function.

In contrast, hyperthyroidism typically presents with weight loss, heat intolerance, and tachycardia rather than bradycardia. Cushing's syndrome is characterized by symptoms related to excess cortisol, such as weight gain, hypertension, and increased blood glucose, but does not specifically cause sensitivity to cold or a low pulse rate. Addison's disease, which involves adrenal insufficiency, may lead to weight loss and fatigue, but would more commonly present with hyperpigmentation and electrolyte imbalances.

Therefore, given the combination of clinical symptoms and their alignment with the pathophysiology of hypothyroidism, this