Im sorry to hear about your symptoms. Now excessiuve fatigue and tiredness can be a feature of a number of diseases. considering the fact that you do have a thyroid disorder it is possible that you may have an accompanying hormonal disorder such as a problem with the adrenal glands. This would result in excessive fatigue, tiredness, low blood pressure at time etc. this is known as adrenal insuffiency and can be diagnosed with the help of a simple clinical examination along with specific tests such as serum cortisol levels among others.
Another cause of this can be related to autoimmune diseases in which the body itself attacks various organs and can result in fatigue, muscle aches, joint pains, tiredness among many other symptoms. this would be diagnosed with varioius tests like ESR, ANA and a complete ENA profile if needed.
lastly if all other causes are ruled out then you can have chronic fatigue syndrome which is a condition in which the major feature is fatigue which is not due to an other definable cause. this would be treated with various drugs including SSRIs among several other options.
Now i would recommend that you first see an internist who can narrow down the problem and then refer you to an endocrinologist or rheumatologist as needed but an internist would be a good option as they are trained in all aspects of medicine and narrow down the list to one after getting the requisite tests done.
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