While it is impossible to say for sure what this mole is without having it biopsied, it think it looks suspicious enough to suggest you see a dermatologist about it. When looking a moles, a good way to determine if they may be melanoma is by using the ABCDE's of melanoma:
A = asymmetry: if you could fold your mole in half, would the two halve pretty much match up? While yours looks mostly round, it's definitely not a perfect circle, so I'm a little suspicious.
B = border irregularity: does the mole have a definite margin, can you tell completely where the mole stops and normal skin begins? If not, it is more likely to be cancerous I can't tell by looking at this picture of yours, so have your parents take a closer look.
C = color irregularity. Is you mole the same color throughout? It is more suspicious if it has 2 or more colors. While yours definitely have at least 2 colors, it does not have an area of central clearing, which would make me most suspicious of a melanoma.
D = diameter >6mm or 1/2 inch. Compairing with the size of your tthumb it looks like your mole may be larger than 1/2 inch in diameter.
E = evolution. Has your mole changed. While moles will grow as you to, their growth should not be out of proportion to your own. Ask your parents about this one, too.
Since you have 3-4 of these signs positive, it think it is a good idea to have your mole checked out. However, on a rreassuringnote, melanomas are generally found on sun exposed areas, and since this mole is on your scalp, it is less likely to be malignant. However, since this is not always the case, get it checked out.
Also, as for the acanthosis nigricans, this is a benign skin change usually noted to be a dark brownish black color and have a vvelvetytexture, and found most often on the back of the neck and in the groin area. It is very commonly seen in people who are overweight, and is associated with insulin resistance, which can lead to type II diabetes. Since you are only 15, it think it would be a good idea to ask you doctor about this as well. You should also make sure your that your hypothyroidism is well controlled by getting your T4 levels (not TSH) checked regularly since you have central hypothyroidism. If you are not already, I think you should be seeing an endocrinologist as well.
Good luck with everything, and I hope this helps.