Hello,
the darkening of the skin could also be due to something called polycystic ovarian syndrome / insulin resistance.
You need to see your Dr about this. if it is due to insulin resistance a drug called glucophage / metformin can be useful.
I've also read that type II diabetes can present with darkening patches of skin.
What type and dosage of thyroid medication are you on?
I have never heard darkening skin as a symptom of thyroid disease. You should see a Dr. Just a guess, but it could be due to an Adrenal issue.