Re: Just heard back about blood test. Still don't really know what it is. Any advice?
I'm sorry you've been feeling unwell and fatigued. It's interesting that your thyroid was "underactive". I wonder what your doctor meant by that. Usually the thyroid when it's underactive, it shows up with a screening test known as the TSH (thyroid stimulating hormone). If the thyroid is underactive, the TSH will be HIGH (yes, paradoxical). Usually if the TSH is found to be high, there is a second set of tests the doctor does to confirm it. Do you know if those were done? Generally, you don't have to wait for 6-8 weeks to check that. However, one thing to look out for (which I think is the reason your doctor is waiting for that long to repeat the test) is that acute illness can mess up your thyroid-related bloodwork and make it falsely abnormal. This usually takes 2-4 weeks to normalize again once you're over the illness.
In terms of your symptoms: unfortunately, something I say often on these boards that your symptoms are very non-specific. They don't point towards any specific diagnosis. That's not to say they aren't troubling! It's just that, with your type of symptomatology, unless something is found initially on screening bloodwork, a cause is often NOT found and it goes away on its own. There is an entity known as "post-infectious fatigue" which can be seen AFTER infection with almost any pathogen (viral or bacterial). In these cases, the infection is long-gone, but the immune system has been stimulated and continues to fire actively despite being "done its job".
To illustrate this: when you have "the flu", the "flulike" symptoms you feel like malaise, nausea, muscle aches, joint pain etc. etc. are actually not from the virus, but from your own immune symptom churning out things called cytokines. These cytokines help to activate the immune system to destroy the invading pathogen. In some people, this churning of cytokines can continue for a few weeks after the infection is gone. This results in many different things, and one of those symptoms is fatigue.
I'm sure your doctor will do more investigations, but given that you're active and otherwise healthy, this will likely go away on its own and you probably will never find a cause.