On 100mcg of Levothyroxine but still feel sluggish, anxious, mental slowness, and lack of motivation. Possible other underlying issues?

Depends. Usually hypothyroid has accompanying labs done. While your dose maybe within range you might need to ask your physician to consider an increase as clinically you are not feeling as good as expected. I usually treat the patient, the labs are guidelines and I would refer you back to whom ever prescribed the med. Need more information as well to consider additional medical diagnosis .