Hi Rick,
Just thinking aloud,
Firstly, does cortisol, (which is made in the blood) cross the BBB into the brain? If it doesn't, and it seems as though it doesn't, it presumably can't affect PD symptoms?
http://www.ncbi.nlm.nih.gov/pubmed/1...m&ordinalpos=7
"entry of cortisol to the brain of these animals is limited at the blood-brain barrier (BBB)"
This is complicated though, since cortisol is released by stress, which also dramatically increases the permeability of the BBB, (and of course increase PD symptoms) Stress may widen the BBB enough to permit passage of cortisol, even though the level is reduced.
However, low cortisol from other causes should show improved PD symptoms if you are correct. What are other causes of higher cortisol levels,(other than stress) and do they cause a worsening of symptoms?

See http://www.ehow.com/about_5184651_ef...l-levels_.html
"Some medications can block the synthesis of cortisol."
So presumably other compounds besides levodopa can cause low cortisol.
I have not had time to find all of them, but one I found was

"One of the best known and most effective ways to lower excess
cortisol levels is with the nutrient Phosphatidylserine"
See http://answers.google.com/answers/th...id/757599.html
I think this is a supplement used to treat PD, so that supports your idea. I must do a search on the others.

A fascinating idea Rick. I wonder what can cause dyskinesia after prolonged use of levodopa. Could a lowering of cortisol be a possible cause?
Dyskinesia must be due to one of the side effects of levodopa. Your reference says,
"
The cortisol concentrations significantly decreased after levodopa intake, particularly in the patients with more advanced stage of PD, but not in the less affected patients. There were significantly lower cortisol levels in the patients at the advanced stage of PD"

Dyskinesia only occurs in advanced patients when levodopa is administered.
I never get diskinesia when I am off and have not taken any levodopa for a long period. So dyskinesia must be a function of taking levodopa. Nothing else causes it.
Sorry if this rambles somewhat.
Ron