- Hypothyroidism is characterized by a decreased metabolic rate due to insufficient production of thyroid hormones.
- These hormones play a crucial role in regulating the cardiovascular system, including heart rate.
- In hypothyroidism, the reduced levels of thyroid hormones lead to a slowed heart rate (bradycardia).

- In contrast, tachycardia (Option 1) is more commonly seen in hyperthyroidism, where increased thyroid hormone levels stimulate the heart to beat faster.

- Similarly, hyperhidrosis (excessive sweating, Option 3) and mydriasis (pupil dilation, Option 4) are signs that are typically associated with hyperthyroid states or sympathetic overactivity, rather than hypothyroidism.

Therefore, the hallmark cardiovascular feature of hypothyroidism is bradycardia, reflecting the overall reduction in metabolic and sympathetic activity.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Endocrinology Chapter, Page 2521