Hello and thank you for your question.
There are many different causes for feeling so tired and you will need to have not only your thyroid levels looked (which is very important and is one of the most common causes of fatigue) but to have blood work to rule out other causes such as anemia (blood loss), kidney/liver problems, and certain vitamin deficiencies (such as Vitamin D). If blood work comes back normal, you might need to have an ultrasound of your heart done (called Echocardiogram) to make sure fatigue is not coming from a weakly functioning heart. You might need to get a sleep apnea test done to make sure you do not have sleep apnea as it is one of the most under diagnosed and under treated conditions.
So, I do agree with the fact that you need a thyroid test done, however, besides thyroid, other things need to be checked for a full work-up of fatigue.
I hope you find this information helpful. Please feel free to follow-up with any other questions you might have.
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Best wishes and good luck,