I've been on finasteride for almost 4 months now after my hair thinned a lot over the last couple of years. (photos)

While it is difficult to say what specific effects the Vitamin D might have had on the hair, finasteride should be used for a minimum of 6 months to see any results.  With most issues related to hair loss or hair restoration, change is slow as hair grows slowly.  Patience is key with any hair restoration or hair loss treatment. Hair transplantation is also a great option for some patients.Best,

It can take up to 6 months to get results with Finasteride and you would need to continue using it to maintain results. I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

Finasteride is an effective drug to stop hair loss and you will see that affect  6 mo after use.But you may have Post-finasteride syndrome, occurs in men who’ve taken finasteride to treat hair loss by a chance of %4-5. That may causes some symptoms include: loss of libido, erectile dysfunction, depression, suicidal ideation, anxiety, panic attacks, Peyronie’s disease, penile shrinkage, gynecomastia, muscle atrophy, cognitive impairment, insomnia, severely dry skin and tinnitus. The condition often has a life-altering impact on victims and their families, such as job loss and the breakup of marriages and romantic relationships..You would better planning to have an exam at hair loss expert for
hair restoration surgery when you fell uncomfortable about hair styling or
total loss of hair..

I have seen many patients with hair loss related to low Vit D but none with hair loss to high Vit D. I am not really sure about your hair loss cause, If you don't have a Male Pattern of Hair Loss (genetic) and you cause could be related to possible "high" Vit D then I would wait for 6-12 months for hair regrowth and I had recommend you to see your PCF and hair loss specialist for further evaluation and diagnostic of your hair loss cause before starting Finasteride. If you had hair thinning before starting to take Finasteride that could be related to androgenetic alopecia and the medication helps you to stable your hair loss and some hair regrowth and you need to take it for long period of time. 

This is a great medication for slowing down hair loss. There is no medication on the planet that will stop hair loss.  You should keep on it and will start to appreciate the positive changes in the coming 3-4 months.  Come in for an in-person consultation so that you can learn about all of your surgical and nonsurgical treatments for ongoing hair loss.