How is hypothyroidism defined?

Hypothyroidism is defined as a condition in which the thyroid gland does not produce enough thyroid hormone. This insufficient hormone production leads to various metabolic slowdowns in the body, affecting processes such as energy regulation, temperature control, and overall metabolism. Individuals with hypothyroidism often experience symptoms such as fatigue, weight gain, sensitivity to cold, and depression, which arise due to the lack of adequate thyroid hormone levels.

The correct understanding of hypothyroidism hinges on its association with low hormone production, distinguishing it clearly from hyperthyroidism, where there is an excess of thyroid hormone (as noted in option A). Normal levels of thyroid hormone production, as described in option C, and abnormal thyroid gland size in option D also do not capture the essence of hypothyroidism, as neither directly addresses the underproduction of hormones that defines the condition.