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Chief Complaint: 54-year-old white female with palpitations.

History of Present Illness: This 54-year-old white female, a computer programmer, presented at her physician's clinic with recent onset of "heart-pounding." She described the episodes as a feeling of fullness in her neck and a racing heart. The events do not seem to be triggered by stress or physical exertion, and have occurred at all times during the day and night. She estimated that they occur about 10 times a day. The palpitations last about 5 minutes and then subside, and are not associated with chest pain or shortness of breath. In addition to the palpitations, she complained of increasing irritability and difficulty concentrating at work, both of which have become noticeable over the past 12 months. She described herself as having always been "the nervous type," but over the past year, a tremor in both hands has made it difficult for her to use a keyboard. In addition, she reported loss of weight in the past three months (116 lbs. to 104 lbs.) despite an increase in her appetite. She complained of mild insomnia and noticed that recently she has been uncomfortable at night when there are lots of covers over her. She denied any history of depression or anxiety disorders, and reported that she does not smoke, drink coffee, or take any diet medications or other prescription or non-prescription drugs.

Past Medical History: The patient has been in relatively good health most of her life. However, she reported an episode of unexplained weakness in her right leg about 5 weeks ago. The symptoms only lasted "a few hours" and then went away. She sought no medical attention at that time. She has not had a recurrence of this since that time.

Family History: The patient's parents are both still alive and in relatively good health (father aged 82 and mother aged 79). One sister, aged 49, is in good health. Both grandfathers and one grandmother died of "natural causes" in old age. The maternal grandmother died at an early age during an influenza epidemic. There is no family history of cancer, heart attack, stroke, rheumatologic, or endocrinologic disease.

Physical Examination: The patient was awake, oriented, and mildly distressed, appearing older than her stated age. Height was 5 ft. 5 in.; weight was 101 lbs. Her skin was warm and diaphoretic (sweaty). There appeared to be generalized muscle wasting, most notably in the proximal limb musculature. Vital signs were as follows: heart rate = 112, blood pressure = 166 / 82 (sitting), respiratory rate = 20, temperature = 100.8^oF (oral). Arterial pulses were bounding in the right and left carotid, brachial, and radial arteries. Right and left femoral, popliteal, and dorsalis pedis pulses were within normal limits. No arterial bruits were heard. There was no jugular venous distension or dependent edema. The heart rate was irregularly irregular, but no murmur, S₃ heart sound, or S₄ heart sound was heard. An enhanced apical impulse was palpated under the left nipple in the 5th intercostal space. Head and neck exam was normal with the exception of a mild proptosis (eye bulging) and upper eyelid lag upon downward glance as well as a slightly enlarged right lobe of the thyroid gland. There were no swollen lymph nodes. Lungs were clear to percussion and auscultation. Abdomen revealed normal bowel sounds and was soft, non-tender, and without masses. Muscle strength, reflexes, and skin sensation were normal throughout the body.

Laboratory Studies: Laboratory studies of the venous blood revealed the following:

Laboratory studies of the arterial blood revealed the following:

A 3-lead ECG was performed in the office at a time when the patient was asymptomatic. The Lead 2 tracing was as follows: