As soon as PTH does its job, PTH needs to be taken out of commission to prevent overdoing it. The only way our body can know if PTH has done its job is by evaluating calcium levels in the blood. When the calcium levels increase, no more PTH should be made. This is an example of negative feedback. In all cases of negative feedback, the end result affects the pathway that got it there.
Most people with primary Addison's have darkening (hyperpigmentation) of the skin, including areas not exposed to the sun; characteristic sites are skin creases (e.g. of the hands), nipple, and the inside of the cheek (buccal mucosa); also, old scars may darken.