Everyone needs lab work done. From thyroid hormones to a CBC and iron levels. Everyone needs to make sure they aren't anemic...and to make sure they aren't hypothyroid before sleep studies. These all sound more like something internal instead of sleep problems.
I too have the whole sleeping too much syndrome and have sort of had it all my life. Now I work night shift and it is worse. My days off are spent sleeping.
I have been diagnosed with depression. I was surprised actually because I didn't think I was depressed, but sleeping too much can be a key sign. I do not take medications, but I have changed how I eat, exercise and I take a multi-vitamin every day and these really seem to help.
I have been trying to use a regular schedule and MAKE myself get up to alarm instead of giving myself the freedom to sleep all day. The first two weeks were brutal. I honestly was useless. I would sit in a chair in the sun, try to read or try to do some laundry. I told myself anything is better than sleeping and it was a cycle I was trying to break.
It worked, honestly. It was as hard if not harder than quiting smoking. Which by the way I used to think made me tired, but after I quit the tiredness continued.
My blood work has all come back normal, for now. They are suspecting pre-menapausal (early at 42). Blood work can change drastically in a very short period of time. (I work in medicine)
Keep trying different things. Try some new sleep hygiene tools such as hot bath, warm milk, hot tub, sauna....NO exercising or caffeine anywhere near your sleep time. Make bed only for sleeping and sex. NO TV, computer, radio....make it dark and cool to trigger melatonin in your brain for more quality sleep. If you can't sleep while you are in bed get up. Go do something else in another room for 10 minutes and try again.
I am in a better place, but still fight this everyday. I wish everyone luck with it!!