For a few years now, I lack energy and strength. What could be wrong?

The short version: your symptoms match several well-documented medical conditions, but the evidence here cannot tell you which one you have. You need lab tests, not more reading.

You described a lack of energy and reduced strength over several years. That is a broad symptom that fits many things. The evidence retrieved covers a range of possibilities, but it is a sample, not a complete workup. The most important thing is that this is not normal aging. Something is wrong, and it is worth finding.

The evidence points to several conditions that cause exactly this. Hypothyroidism is a strong candidate. One case report describes a 34-year-old man with profound muscle weakness, sky-high creatine kinase (CK) at 7,332 IU/L, and a TSH over 100, who improved dramatically after thyroid hormone replacement [4]. Another case shows celiac disease can present with proximal muscle weakness and rickets, not just gut symptoms [3]. Statins can cause a rare but severe autoimmune muscle destruction called necrotizing myopathy, with CK levels over 10,000 and weakness that persists after stopping the drug [5, 6, 7]. Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is another possibility, where energy is limited by a clear biological mechanism: post-exertional malaise and impaired energy metabolism [1, 2]. One study found that people with ME/CFS who stayed within their "energy envelope" improved fatigue by 16% and physical function by 37.6%, compared to 2.7% and 6% in those who pushed past it [2].

The evidence does not tell you which of these you have. That requires a doctor who will run the right tests: TSH, free T4, free T3, CK, celiac antibodies, and a careful history for statin use or ME/CFS triggers. The evidence also does not include long-term safety data for any treatment for these conditions. The case reports show that treating the underlying cause (thyroid hormone, gluten-free diet, stopping the statin) can reverse the weakness, but these are single cases, not large trials [3, 4, 5, 6].

My call: the evidence supports several plausible diagnoses, but you need lab work to know which one applies to you. Confidence: moderate. The evidence is strong for the mechanisms but thin on how often each cause appears in someone like you.