I overdosed on caffeine before i was diagnosed with hypothyroidism. i can't sleep. could it be my serotonin levels and melatonin levels dropped?

Partly.: Caffeine itself can be the culprit for your insomnia. Hypothyroidism can cause depression which can lower your serotonin level and can contribute to your insomnia. Melatonin is a diet supplement that can improve the quality of your sleep. In the winter time when we have less exposure to sunlight, our melatonin level can go down which contributes to the winter blues and insomnia.