Hi Kat,
I don't have any info on studies or causal theories; however, I do have some experience with a parathyroid adenoma and having an AN...
Little history,, I was diagnosed with thyroid nodules in about 1975 and chose to just monitor them...(no meds) ,, I was followed by an endocrinologist for years and in 2004 an ultrasound indicated that one nodule (out of 8 I had been monitoring) had calcified and thyroid cancer was diagnosed. During thyroidectomy surgery, it was discovered that one of my parathyroid glands was enlarged and had to be removed. Then in 2008 I was diagnosed with my AN.. coincidentally, or not,, all nodules/tumors were on my right side..
Are they related?? Who knows,, but it's difinitely interesting,,, keep us informed if you find out more info on this ,, some of us are extremely interested,, good luck to you and keep your nodules monitored closely. Jane