What is the SINGLE most likely diagnosis for a woman with tiredness, cold intolerance, and a history of Addison's disease?

The most likely diagnosis in this case is hypothyroidism. The symptoms of tiredness and cold intolerance align closely with what is typically observed in individuals suffering from hypothyroidism, a condition characterized by an underactive thyroid gland which leads to reduced production of thyroid hormones. These hormones are crucial for regulating metabolism, energy levels, and temperature control. Individuals with this disorder often experience fatigue and heightened sensitivity to cold due to a decreased metabolic rate.

In this scenario, the history of Addison's disease – an adrenal insufficiency disorder – further supports the diagnosis of hypothyroidism. Patients with autoimmune conditions are at a higher risk of developing other endocrine disorders, including those affecting the thyroid. Therefore, the combination of tiredness, cold intolerance, and concurrent Addison's disease points toward an underactive thyroid as the likely cause of the symptoms presented.

Other conditions like polycystic ovary syndrome, hyperthyroidism, and premature ovarian failure present with different symptoms and do not align as closely with the given symptoms of tiredness and cold intolerance, making hypothyroidism the most fitting diagnosis.