Final answer:
A patient with a non-functional thyroid gland likely suffers from hypothyroidism, which causes symptoms like lethargy and mental sluggishness due to low thyroid hormone production.
Step-by-step explanation:
A patient with a completely non-functional thyroid gland is likely to experience lethargy and mental sluggishness. This condition, known as hypothyroidism, results from the underproduction of thyroid hormones and leads to symptoms such as a low metabolic rate, weight gain, sensitivity to cold, and reduced mental activity. Other symptoms can include constipation, reduced libido, menstrual irregularities, and fatigue. In contrast, hyperthyroidism, which is caused by overproduction of thyroid hormones, would show symptoms such as weight loss, excess heat production, sweating, and an increased heart rate. Therefore, the correct answer is b. Lethargy and mental sluggishness.