The differential diagnosis for fatigue is just so vast, it's easy to miss the marginal percent cases like various forms of hyper/hypothyroidism (yes, both can cause fatigue; I was tested for both, among many other tests.) Unless you have a doctor that's fairly dedicated to solving your case, it often just doesn't happen - the patient's have to do a hell of a lot of the work in a lot of these cases. You
must be your own advocate to get the doctors to run the right tests, try the right drugs... it's probably the only reason drug advertising is ever legal at all.
I live with an autoimmune disease that went undiagnosed for nearly 8 years (I was about 14 when the muscle and joint pain began, some short amount of time after a nasty flu that I personally wasn't sure I'd get through). I had no idea what was wrong but I knew it had to be something, despite what everyone else was trying to tell me. Doctor after doctor would come up with the wrong answer - growing pains ("they'll go away on their own in a few months/years"), shin splints, faking it for drugs (you know, while my mom had to tie her teenage son's shoes because he was physically incapable of bending at the knees through swelling and pain to tie them - eventually switching to flip-flops, even through Kentucky winters), overexertion, various vitamin deficiencies (which they didn't actually test for, just said "take a multivitamin")... Some didn't even bother attempting to come up with a solution, instead just offering opiates or steroids to get rid of me - it was never a surprise to me that there would be an opiate crisis in this country with how I was treated.
It wasn't until after I was taking borderline poisonous amounts of naproxen and suffering horrible gastric symptoms that a clinic doctor finally realized that I wasn't faking it, and nobody in all of those years ran even the simplest of blood tests - an Antinuclear Antibody test. It came back off-the-chart positive, all the way through the highest dilution the lab could test (something like a 1:2000 dilution, with a speckled pattern), and that was what it took to be sent on to a rheumatologist and an enormous battery of tests to eventually get the diagnosis and treatment I needed. Autoimmune diseases are rare in men - my particular disease, Mixed Connective Tissue Disease, is often noted as about 16:1 favored towards women. Autoimmune diseases are rare in the population as a whole, and overlap syndromes are the hardest to diagnose. Multiplicatively speaking, the doctors weren't wrong to rule it out at first - my case is somewhere between 0.5-1 in a million. But by failing to do even the simplest blood test for so long, I suffered various forms and levels of agony, needlessly.