The most likely diagnosis is fibromyalgia, which is characterized by chronic widespread pain, tenderness of skin and muscles to pressure (allodynia), fatigue, sleep disturbance, and exercise intolerance. Previous lack of response to multiple medications, including NSAIDs, also provides a diagnostic clue. Examination is generally unremarkable except for allodynia. An association with other pain syndromes, including irritable bowel syndrome, irritable bladder, pelvic pain, vulvodynia, headache, and temporomandibular jaw pain, is not uncommon. This patient fulfills the 2010 American College of Rheumatology diagnostic criteria for fibromyalgia (widespread pain, wakes unrefreshed, significant fatigue, and cognitive difficulties), with symptoms present for more than 3 months.

The clinical manifestations of adrenal insufficiency are often insidious, with fatigue and malaise being the dominant symptoms; a high degree of clinical suspicion may be needed to pursue the diagnosis in the presence of subtle systemic symptoms. The patient's 4-year history of symptoms and a normal chemistry panel makes adrenal insufficiency unlikely.

The numerous and largely nonspecific clinical manifestations of hypothyroidism include fatigue, reduced endurance, weight gain, cold intolerance, constipation, impaired concentration and short-term memory, dry skin, edema, mood changes, depression, psychomotor retardation, muscle cramps, myalgia, menorrhagia, and reduced fertility. Some patients with mild hypothyroidism will exhibit few or none of these symptoms. A normal thyroid-stimulating hormone level makes hypothyroidism very unlikely.

The classic findings of polymyositis are symmetric proximal muscle weakness with little or no pain and elevation in muscle-associated enzymes. This patient has pain, a normal serum creatine kinase level, and no true weakness on examination, making polymyositis an unlikely diagnosis.