Unfortunately, a lot of doctors will tell their patients to take 1000 iu of Vitamin D per day, and justify that by saying the US RDA is 600 iu per day or 800 for people above 70. They learned in medical school that Vitamin D is fat soluble, so can accumulate in the body and cause calcification in the organs.
Research of the last 10 years says worries about Vitamin D overdose are vastly overstated. People have accidently been given 1000 times the RDA for months, with no ill effect. In fact, the calcification problem is caused by too much calcium in the blood!
The US RDA was set back when most people worked outside and got lots of sun. Sun exposure of half of the time that would cause sunburn gives you about 20,000 iu. That's 33 times the USRDA! The Endocrine Society says 50 ng/ml is optimal. That would require around 5000 iu per day for most people.
1000 iu/day will give you a blood level of 10, absent any summer sun exposure or oily fish consumption. That's a lethal deficiency if you catch Covid-19.
