Patient description: The patient is a 33-year-old woman complained of fatigue, weight gain and abnormal spotting between menses. No hirsutism or nipple discharge was detected. Her BMI was 34. Her lab results were remarkable for high TSH level (13 mU/L) and low free T4 level (0.2 ng/dl). Her anti-TPO levels were extremely high (120 IU/ml). She was diagnosed with Hashimoto's thyroiditis. Her aunt, brother and mother have the same disease. After starting 250 mcg Levothyroxine per day, her symptoms improved significantly and her periods are normal. She is still overweight with BMI of 31. Her most recent thyroid profile revealed all results except for anti-TPO within the normal range: TSH: 2.35 mU/L Free T4: 2.7 ng/dl Anti-TPO: 75 IU/ml

Clinical trial eligibility criteria: eligible ages (years): 18.0-60.0,  Obesity Diet Therapy Weight Loss Weight Gain Nutrition Therapy BMI greater than or equal to 27 kg/m(2) for the weight loss protocol but body weight less than 350 pounds to accommodate the DXA scanner BMI less than or equal to 24 kg/m(2) (and BMI greater than or equal to 18.5 kg/m(2) for the overfeeding protocols. A history of low BMI and difficulty gaining weight Age 18-60 years, to minimize potential co-morbid conditions which may indirectly affect EE. Minors under the age of 18 will be excluded because growth and pubertal issues are significant parameters that could affect our outcomes and also because the time requirements of the study are such that they would interfere with school schedules. Women who are post-menopausal will be excluded from the study as changes in their metabolism could affect the baseline measurements which are hypothesized to predict weight change Healthy, as determined by medical history, physical examination, and laboratory tests  Current smoking Type 2 diabetes (according to the World Health Organization diagnostic criteria) Impaired glucose tolerance (according to the World Health Organization diagnostic criteria) for those participating in the overfeeding study arms only Endocrine disorders (Cushing s Disease, pituitary disorders, and hypo and hyperthyroidism) Chronic pulmonary disorders, including chronic obstructive pulmonary disease that would limit ability to follow the protocol (investigator judgment) and obstructive sleep apnea syndrome; only subjects with mild or exercise-induced asthma on no medications or on beta-adrenergic agonists only (such as albuterol) will be allowed to enter the study (provided use of these agents is not required for one week before study entry) Cardiovascular diseases (coronary heart disease, heart failure, arrhythmias, and peripheral artery disease) Hypertension (blood pressure measurement higher than 140/90 mm Hg (overfeeding study) or 160/95 (weight loss study) on two or more occasions or use of anti-hypertensive medications) Diagnosed gastrointestinal diseases, including inflammatory bowel diseases (e.g., Crohn s disease and ulcerative colitis), malabsorption syndromes (e.g., celiac disease), gastric ulcer (active); only subjects with gastro-esophageal reflux will be allowed to enter the study Presence of a pacemaker or other implantable devices/shrapnel which may interfere with the MRI or CorTemp measurements