Final answer:
Hypothyroidism is likely caused by a shift of thyroid hormone in the blood, whereas hyperthyroidism is characterized by an abnormally elevated blood level of thyroid hormones.
Step-by-step explanation:
Hypothyroidism is likely caused by a shift of thyroid hormone in the blood form the protein-bound (biologically inactive) to the free (biologically active) state. It is characterized by a low metabolic rate, weight gain, cold extremities, constipation, reduced libido, menstrual irregularities, and reduced mental activity. On the other hand, hyperthyroidism is characterized by an abnormally elevated blood level of thyroid hormones. It can be caused by a pituitary or thyroid tumor or by autoimmune reactions like Graves' disease. Hyperthyroidism leads to an increased metabolic rate, excessive body heat and sweating, diarrhea, weight loss, tremors, increased heart rate, and bulging eyes.