Hypothyroidism: Although the TSH level is within the normal range, the patient's significant weight gain and fatigue could be indicative of hypothyroidism, especially considering the T3 and T4 levels are on the lower end of the normal range. Some individuals may experience symptoms of hypothyroidism even with normal TSH levels, particularly if they have a history of thyroid issues or other underlying conditions.

Insulin Resistance/Pre-diabetes: With an HbA1C of 5.5, the patient is at the higher end of the normal range, suggesting possible insulin resistance or pre-diabetic state, which could contribute to weight gain and fatigue.
Polycystic Ovary Syndrome (PCOS) in Males (or equivalent condition): Although PCOS is typically associated with females, males can exhibit similar metabolic and hormonal imbalances, leading to weight gain, fatigue, and potentially altered thyroid function tests.
Cushing's Syndrome: This condition, characterized by excess cortisol, can lead to significant weight gain, particularly in the central part of the body, and fatigue. However, it would typically present with other symptoms such as hypertension, glucose intolerance, and specific physical findings.

Pituitary Adenoma: A tumor in the pituitary gland could affect hormone production, including thyroid-stimulating hormone (TSH), leading to symptoms like weight gain and fatigue. It's crucial to consider this diagnosis due to its potential impact on the patient's health if left untreated.
Sleep Apnea: Given the patient's significant weight gain and fatigue, sleep apnea is a critical condition not to miss. It can lead to daytime fatigue and has been linked to various metabolic and cardiovascular diseases.
Depression: Depression can manifest with significant weight gain or loss and fatigue. It's essential to consider mental health diagnoses, especially given the potential for overlap with physical symptoms and the impact on the patient's quality of life.

Craniopharyngioma: A rare type of brain tumor that can affect the pituitary gland and hypothalamus, leading to hormonal imbalances, weight gain, and fatigue.
Prader-Willi Syndrome: A genetic disorder that can cause a constant hunger feeling, leading to overeating and obesity, among other symptoms. It's rare but should be considered in cases of unexplained significant weight gain.
Leptin Deficiency: A very rare condition where the body cannot produce enough leptin, a hormone that helps regulate energy balance, leading to severe obesity and potentially other metabolic issues.