I have read studies in the past of the importance of vitamin D for test production especially in the winter time. I always try to up my vitamin D supplementation during pct but like most things in the human body it's about finding balance. Here is an interesting study about vitamin D and how having too much can cause the very thing you are trying to prevent.
https://www.ncbi.nlm.nih.gov/m/pubmed/25044703/?i=2&from=/20050857/related
https://www.ncbi.nlm.nih.gov/m/pubmed/25044703/?i=2&from=/20050857/related