I am 42 years old and overweight. but i am also constantly tired, as if i do not get enough sleep. my thyroid has been checked, but it is functioning normal. what could it be?

Too many: From anemia or low blood to sleep apnea, a variety of conditions may account for your symptoms. Best would be to book an appointment with your GP for an exam and further tests as needed.

Need to examine: It is not feasible to provide a meaningful opinion without taking additional history, physical examination and may be some tests. It would be prudent to see your doctor and discuss if you may be depressed. Counter-intuitive as it may seem, exercise may help. Start with low intensity exercise such as walking and gradually increase the intensity as tolerated. Wish you good health!

See a good doctor!: There are many possible causes of your symptoms: many type of infections (bacterial, viral, fungal, parasitic), anemia, nutrient deficiencies,toxic exposures,thyroid & other hormonal problems,sleep disorders,heart disease,stress, depression etc. It's impossible for anyone here to diagnose you further. See a good doctor, preferably with a holistic perspective, for a thorough evaluation and comment.