There are many good articles published on cyclical Cushing's. You wouldn't want to restrict your search to NIH.
Dr. Stratakis of NIH published the following paper:
Cyclical Cushing Syndrome Presenting in Infancy: An Early Form of Primary Pigmented Nodular Adrenocortical Disease, or a New Entity?
NIH published the following paper concerning screening tests with author Dr. Nieman and others.
I found the following publications for you using PubMed for the search. BTW, Google is not your friend for starting these medical topic searches. Google will lead you in too many directions and information that hasn't been peer reviewed often has mistakes.
Search on PubMed. If you find an interesting title that only has the abstract, you can often find the full text by pasting the full title into google. Weed through the paid document providers and you'll often find a free full .pdf document later in the list.
Sometimes you can do a search directly on a publisher's web site for even more articles.
Here are some articles for you. I encourage you/your friend to spend s few evenings searching and reading.
Cyclical Cushing’s syndrome due to an atypical thymic carcinoid
There's also lots of valuable information on Dr. F.'s (Theodore C. Friedman, M.D., Ph.D.) website.
I recommend you spend an entire evening reading on Dr. F.'s web pages. His work integrates clinical experience that's lacking in much of the other work. Then read everything Dr F. published by using www.pubmed.com with an author search. On pubmed you do an author search by typing F. T [au] in the search box.
When you find a useful article on pubmed, click on the authors name in the author list below the title. That will bring up a listing of their other publications. Often these authors have a special interest in the topic you're searching and have published many other papers. If there is a "free text" icon shown to the upper right, you can read the full article by following the link. If the free icon isn't there, copy and paste the article title into google. You may get lucky if you spend some time.
Concerning testing, the endocrine society has published a guideline that represents current practice.
You have the right approach. An educated patient can get through the diagnostic maze by understanding the limitations of the classical tests. A patient who doesn't know the research is likely to be screened out and never get the necessary care.
Cyclic Cushing's is a controversial subject among endocrinologists. Other terms used for it are "episodic" and "intermittent". Tumors which cause Cushing's disease can intermittently secrete hormones which cause increased cortisol.
Pituitary tumors will secrete ACTH intermittently while adrenal tumors secrete cortisol periodically. This leads to a cycle of Cushing's symptoms which may repeat over the period of hours, or days, or weeks, and sometimes even years. Because of this, it is difficult to diagnose for some patients.
To review some articles which include cyclic/episodic/intermittent Cushing's, see the links below: