When Ernest Eaton's desperate wife finally got him to agree to a comprehensive inpatient evaluation, he was 37, unemployed, and had been essentially nonfunctional for several years. After a week during which ha was partying all night and shopping all day, Mrs. Eaton said that she would leave him if he did not check into a psychiatric hospital. The admitting psychiatrist found him to be a fast-talking, jovial, seductive man with no evidence of delusions or hallucinations. Mr. Eaton's troubles began 7 years before when he was working as an insurance adjuster and had a few months of mild, intermittent, depressive symptoms, anxiety, fatigue, insomnia, and loss of appetite. At the time, he attributed these symptoms to stress at work, and within a few months was back to his usual self. A few years later an asymptomatic thyroid mass was noted during a routine physical exam. One month after the removal of the mass, a papillary cyst, Mr. Eaton noted dramatic mood changes. Twenty-five days of remarkable energy, hyperactivity, and euphoria were followed by five days of depression during which he slept a lot and felt that he could hardly move. This pattern of alternating periods of elation and depression, apparently with few "normal" days, repeated itself continuously over the following years. During his energetic periods, Mr. Eaton was optimistic and self-confident, but short- tempered and easily irritated. His judgment at work was erratic. He spent large sums of money on unnecessary and, for him, uncharacteristic purchases, such as a high-priced stereo system and several Doberman pinschers. He also had several impulsive sexual flings. During his depressive periods, he often stayed in bed all day because of fatigue, lack of motivation, and depressed mood. He felt guilty about the irresponsibility and excesses of the previous several weeks. He stopped eating, bathing, and shaving. After several days of this withdrawal, Mr. Eaton would rise from bed one morning feeling better and, within two days, be back at work, often working feverishly, though ineffectively, to catch up on work he had let slide during his depressed periods. Although both he and his wife denied drug use other than drinking binges during his hyperactive periods, Mr. Eaton had been dismissed from his job five years previously because his supervisor was convinced his overactivity must be due to drug use. His wife had supported him since then. When he finally agreed to a psychiatric evaluation two years ago, Mr. Eaton was minimally cooperative and noncompliant with several medications that were prescribed. including lithium, neuroleptics, and antidepressants. His mood swings had continued with few interruptions up to the current hospitalization. In the hospital, results of his physical examination, blood chemistry, blood counts, CT scan, and cognitive testing were unremarkable. Thyroid function testing revealed some laboratory evidence of thyroid hypofunction, but he was without clinical signs of thyroid disease. After a week, he switched to his characteristic depressive state. What is the most appropriate diagnosis and your detailed rationale for the same?