What is a common consequence of unmanaged hypothyroidism?

Unmanaged hypothyroidism leads to a slowed metabolism due to insufficient production of thyroid hormones. This decrease in metabolic rate often results in weight gain, as the body's ability to properly utilize energy from food diminishes. Individuals may find it difficult to lose weight even with diet and exercise because their metabolism does not function efficiently. Additionally, other symptoms such as fatigue, cold intolerance, and depression may accompany this condition, further contributing to lifestyle changes that can promote weight gain.

In contrast, fast metabolism and increased energy levels are typically associated with hyperthyroidism, where excessive thyroid hormones elevate metabolism. Low blood pressure can occur in certain contexts but is not a primary consequence of hypothyroidism. Thus, weight gain is a direct and common consequence of untreated hypothyroidism, making it the correct answer.