A lost energy can be related to the thyroid gland. But if your thyroid blood levels are normal (thyroid function tests; T3, T4, TSH), some other cause should be looked for the lost energy. Following investigations are needed to assess the cause;
1) Complete blood cell (CBC) count and differential count, 2) Basic metabolic panel, 3) Urinalysis, 4) Thyroid-stimulating hormone: Hypothyroidism can cause diffuse muscle pain and fatigue. 5) Creatinine phosphokinase (CPK) to rule out inflammatory myopathies 6) Erythrocyte sedimentation rate (ESR)7) Antinuclear antibodies (ANAs) 8) Rheumatoid factor9) Sleep studies
10) Adrenal function tests.
Fibromyalgia, lupus, sleep disorders etc are the diagnostic consideration for the fatigue or lost energy. If these are ruled out, other consideration would be;1) chronic fatigue syndrome; unexplained, persistent or relapsing fatigue with unrefreshing sleep, muscle pain, concentration problems, headache, sore throat etc. 2) idiopathic fatigue; it does not meet the above criteria.A consultation with either a rheumatologist or a clinical immunologist would be prudent for an evaluation.
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I would be happy to assist you further, if you need any more information.