Certainly you can get tests from a doctor. I'd evaluate thyroid too.
Early graying can be from low folate intake, and often occurs with thyroid disease. The goiter is a rather obvious sign of thyroid trouble too. I had a right sided goiter, due to the fact my left lobe of the thyroid was not functioning properly. It receded when I was given levothyroxine.

pigmentation in those locations can also be from insulin resistance:
http://www.aocd.org/skin/dermatologi...s_nigrica.html

My mother had this discoloration, and it is not so dark in Caucasian people. She had diabetes.

I, on the other hand, with insulin resistance for many years, do not have this.

Another cause of pigmentation of the skin is hemochromatosis:
This link shows the bronzing skin color in a light skinned woman who was misdiagnosed as Melasma (mask of pregnancy).
She in fact had hemochromatosis:

http://www.blisstree.com/articles/mi...tion-shift-28/
Hemachromatosis can cause neuropathy.

Skin pigmentation also may occur with tinea versicolor, which is a mild fungal skin condition.

So there are many causes.

Lower back pigmentation may occur with spina bifida or spina bifida occulta.

So a good evaluation by a doctor would be in order, to accurately determine if your bronzing is adrenal or another cause.