What condition results from an underactive thyroid?

An underactive thyroid is indicative of hypothyroidism, a condition where the thyroid gland doesn't produce enough thyroid hormones to meet the body's needs. These hormones are crucial for regulating metabolism, energy levels, and overall bodily functions. Symptoms of hypothyroidism can include fatigue, weight gain, cold intolerance, dry skin, and hair loss, among others.

In this context, hypothyroidism best describes the consequence of an underactive thyroid, as the body lacks the essential hormones that facilitate metabolic processes. Other options like hyperthyroidism refer to an overactive thyroid that produces excess hormones. A goiter is an enlargement of the thyroid gland and can occur in both hypo- and hyperthyroidism as it can be a compensatory response to insufficient hormone production or an indication of hormone imbalances. Cushing's syndrome, on the other hand, is related to excess cortisol production from the adrenal glands, entirely separate from thyroid function. This makes hypothyroidism the correct answer in understanding the implications of thyroid underactivity.