For a few years now, I lack energy and strength. What could be wrong? When extensive lab work was done nothing untoward was found.

The short version: your "normal" lab results may be missing what is actually wrong. The standard thyroid panel often misses low T3, and vitamin D deficiency is a proven cause of muscle weakness and fatigue that routine labs sometimes skip.

The evidence here points to two specific blind spots in standard testing. A 2025 case series found that adding liothyronine (T3) to standard levothyroxine resolved fatigue in hypothyroid patients whose TSH was normal but whose T3 was low [1]. A 2019 review confirms that persistent symptoms on levothyroxine alone are "incompletely understood" and that no progress has been made on the research questions that would clarify them [2]. The standard test checks TSH and sometimes free T4, but rarely total T3. If your thyroid is not converting T4 to T3 efficiently, your TSH can look fine while your cells starve for active hormone.

Vitamin D deficiency is another documented cause. A 2010 meta-analysis found that vitamin D supplementation improves proximal muscle strength specifically in people who are deficient [5]. A 2025 study of long COVID patients found that 99 out of 155 reported fatigue, and new diagnoses included hypothyroidism and vitamin D-related conditions [3]. A 2002 case report showed that celiac disease can present as proximal muscle weakness with vitamin D deficiency and rickets [4]. If your labs did not include 25-hydroxyvitamin D, total T3, and a celiac panel, they were not "extensive" enough to rule out the common reversible causes of chronic low energy.

My call: the most likely explanation is a deficiency or conversion problem that standard labs miss. Ask for total T3, 25-hydroxyvitamin D, and a celiac panel. Confidence: moderate, because the evidence here is limited to case series and meta-analyses, and your specific labs are unknown.