Michael is a thirty year old salesman who spends approximately 4 days each week traveling to visit with customers in his region. During his routine physical he casually mentions to his physician that he seems to be sweating more profusely than normal and most rooms that once were comfortable are now too "hot". Michael also reports that he seems to be losing weight even though his appetite has increased. He also complains that he has a shortened attention span and that he always wants to be moving around. Despite the fact that he feels fatigued, Michael claims to have difficulty sleeping and seems to have more frequent bowel movements, occasionally accompanied by diarrhea. The physician checks Michael's medical history and finds that indeed he has lost 15 pounds since his last physical.

Results of Michael's physical examination and tests performed by endocrinologist concluded that Michael had Grave's disease, a form of hyperthyroidism believed to be autoimmune in nature. Michael was presented with a number of possible treatment options. After considering all the options, especially the possible effects of radiation on gamete development, Michael chose surgical removal of the thyroid gland. Following successful surgery, Michael was prescribed synthetic thyroid hormone to ensure that his body was receiving adequate thyroid hormone and told to return within 2 months for a follow-up evaluation of circulating thyroid hormone concentrations. He was also cautioned to carefully monitor his calcium intake.

a. Thyroid hormones exert their effects on cells in a manner similar to steroid hormones; describe the mechanism of action of thyroid hormones.

b. Why would an imbalance in thyroid hormones have such widespread effects on the body?