Final answer:
The condition where the thyroid is underactive and doesn't produce enough hormones is called hypothyroidism, leading to symptoms like weight gain and fatigue due to a slower metabolism. The correct option is b.
Step-by-step explanation:
When the thyroid is not active enough and does not produce enough thyroid hormone, the condition is known as hypothyroidism. Hypothyroidism is a result of insufficient production of thyroid hormones T3 and T4, which are crucial for regulating the body's metabolism.
Reasons for this underactivity can include diseases of the thyroid gland, issues with the pituitary gland or hypothalamus, or a deficiency of iodine.
Patients with hypothyroidism, like Patient A, typically experience symptoms such as weight gain, cold sensitivity, low heart rate, and fatigue, indicating a decreased metabolism. Option b. is the correct one.