are the blackouts when you stand up from a sitting or lying position? When you say extreme weakness is it also fatigue so bad you can not do normal activities like climb stairs or some days can hardly get out of bed? With the nausea do you have an all over feeling of just not feeling good like you're coming down with something? Is your thyroid problem by any chance hoshimotos, an autoimmune disease? You may also have darkening of your skin.
I have Addisons disease and these are all the symptoms I had. It took a little while for a dx. It is considered a rare disease. It basically is your immune system attacking your adrenal glands. If that's the case you take a pill to replace the hormone it produces. I also have lupus so mine is a lot more complicated.
I believe there is a community here for low adrenal function. Hope you find the help you need soon
Check out www.immed.org and look under Autoimmune illnesses. This researcher has over 400 peer reviewed and accepted papers, 4 world renowned citation papers and is on several medical institution/research boards. His name is Dr Garth Nicolson and he has found Mycoplasmal Fermentans-Incognitus to be causing alot of autoimmune illnesses along with other chronic intracellular bacterial infections. THAT is the reason ALL tests have been negative. Also think Lyme disease. Your friend probably had a few Lymes tests done. The Labcorp and Quest tests are worthless. They only look for antibodies (your immune system is disabled, you wont show any). The IGENEX test looks for the actual bacteria DNA in your blood, and is much more sensative.
The blackouts can be at any time or activity. As to nausea, actual stomach pain to the point of nausea. The weakness doesn't allow her to perform normal functionality. Extremely healthy 6 months ago, running 4 miles a day, to unsteady movement and unable to walk much of the time.