Although this is slightly off topic in a thyroid forum it does has some relevance as vitamin D deficiency is common in hypothyroidism. This study journals.plos.org/plosone/a... shows that vitamin D levels below 60 ng/ml (150 nmol/L) are associated with increased breast cancer risks. The solid black line in Fig 1 clearly shows the incidence rates, the extra risk associated with low vitamin D status. I found this study via the Vitamin D Council website vitamindcouncil.org/ . Although this organisation is biased in the sense they are Vitamin D advocates they do produce reliable evidence-based information.
Low vitamin D status is also associated with risks of other cancers whilst excess vitamin D is harmful to calcium and magnesium status which can affect bone and cardiac health. Like all hormones too much or too little vitamin D seems to be harmful. It’s a case of appropriate levels rather than getting levels as high as you can.
I’m not expert in this field, it’s over twenty years since I read up on vitamin D but the increased risk of breast cancer in subjects with low vitamin D levels is quite striking and of particular interest to hypothyroid patients.