Dehydration is definitely a good place to start and making sure you are getting enough water is always important. (A good rule of thumb is to take your body weight in pounds, divide it by 2, then drink that number in ounces per day. Ie: 180lbs/2=90 90/8oz=11.25 cups of water a day. Add more as needed for heavy work out days!)
I also have depression and fibromyalgia which make me think i want to sleep all day so it's a good thing to get checked for them as we'll as get your thyroid checked. (Make sure they do the full spectrum, not just the first two or they may miss something)
Another recent study has shown that 8 hours of sleep can be too much sleep for some people! Getting too much sleep can make you as tired as not enough. Try cutting it back by 30 minutes each week until you find your 'energy' zone. Example: this week go for 7.5, next week go for 7, then 6.5. I found mine to be between 6.5 and 7. Any more than that and I could sleep all day and never feel rested!
If none of this works, also look to your local doctor/clinic for a sleep study to make sure you can rule out sleep apnea or other sleep disorders.
Other tips:
As soon as you get up, drink a 16oz glass of cold water. By that I mean you do this FIRST.
Reduce caffeine (try for just your normal cup only in the morning and no caffeinated beverages after noon.). Another side effect of reducing caffeine to only once per day is that you will be more sensitive to it and it may start giving you that "jump" to your day. Many people have turned to warm lemon water with coconut oil in it instead of coffee as their first drink of the day and have had great results.
Try not to eat after 7pm. Try keeping a dinner/sleep diary. Notice any trends? Do you sleep better on lighter dinners or heavier?
Avoid drinking alcohol to make yourself tired. It may make you feel sleepy, but studies show that it can lead to very disruptive sleep after it wears off.
Exercise!! One of the fastest ways to start sleeping better is to increase your exercise. Which is annoying advice I know because your chief complaint is being too tired to do anything! However, you will notice almost an immediate change.
More Vitamin D! Try to get outside and do something you enjoy for a minimum of 15 minutes. If you go outside on your lunch break at work and go for a short walk you will get a great refresher for the second half of your day!