I've seen multiple sources say that 1000 IU of vit D is usually insufficient to boost levels, with suggestions of up to 5000 a day. My mothers doctor put her up to 2000 a day because 1000 did nothing to her levels, which were already low.
https://pubmed.ncbi.nlm.nih.gov/30611908/
> In summary, long-term supplementation with vitamin D3 in doses ranging from 5000 to 50,000 IUs/day appears to be safe.
https://www.mayoclinicproceedings.org/article/S0025-6196(15)...
> The evidence is clear that vitamin D toxicity is one of the rarest medical conditions and is typically due to intentional or inadvertent intake of extremely high doses of vitamin D (usually in the range of>50,000-100,000 IU/d for months to years).
You can find more sources by scrolling down to the "summary" section, https://www.foundmyfitness.com/episodes/jre-1474
> The most accurate way to measure how much vitamin D is in your body is the 25-hydroxy vitamin D blood test.