Quick Google results:

- Mayo Clinic says "Taking 60,000 international units (IU) a day of vitamin D for several months has been shown to cause toxicity."

- news-medical.net says "To prevent toxicity, tolerable upper intake levels have been fixed for vitamin D as..." 4000 for adults.

- healthline.com says "A daily vitamin D intake of 1000–4000 IU (25–100 micrograms) should be enough to ensure optimal blood levels for most people."

https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...

https://www.news-medical.net/health/Vitamin-D-Overdose.aspx

https://www.healthline.com/nutrition/how-much-vitamin-d-is-t...

Studies of pale skinned Norwegiens who go in the sun during summer show they make 10,000-20,000IU equivalent a day. Results will vary depending on skin tone and the position of the sun (lower in the sky means no vitamin D as the right wavelengths of light are not hitting your skin)

https://www.healthline.com/nutrition/vitamin-d-from-sun#time...

The safest way is to take an occasional test for your vitamin d levels. There are tests that you can take at home that aren't too expensive!

Yep. I work inside a lot, so I started taking 5000IU of D a day. After two weeks, I had my levels tested and it showed a level of 37. When fall comes around, and I'm outside even less, I'll probably up the dosage for a week or two, then test again. Target is between 50 and 70, so I definitely need more.

Thanks. I didn't know.

Amazon has a few options in the $50-90 range.

You prick your finger, draw a "small amount" of blood, and send it to a lab.

The order it yourself lab companies compete on the Vitamin D test, you can usually get one for $40.