My theory is that people with 1,25(OH) toxicity have already high 1,25(OH) levels for some reason or cholecalciferol is metabolized by CYP2D6. I’m slow metabolizer, and if any med I’m taking is metabolized by CYP2D6, I got toxicity symptoms since day one. I need to take a quarter of the therapeutic dose so my body could use this medicine correctly. On the other hand, fast metabolizers need to take 4x of the therapeutic dose. It would explain why doses of D3 varies so much in people’s statements. Of course, it’s a fat-soluble vitamin, so weight is an important factor when you adjust the dose, but the pace of your’s liver metabolization is even more important IMO.
The main symptoms of vitamin D overdose which are those of hypercalcemia including anorexia , nausea, and vomiting. These may be followed by polyuria , polydipsia , weakness, insomnia, nervousness, pruritus and ultimately renal failure . Furthermore, proteinuria , urinary casts , azotemia , and metastatic calcification (especially in the kidneys) may develop.  Other symptoms of vitamin D toxicity include mental retardation in young children, abnormal bone growth and formation, diarrhea, irritability, weight loss, and severe depression.  
The best source of Vitamin D is safe, regular exposure to sunlight. Doctors recommend exposing the face and hands to direct sunlight at 10 minutes at a time at least once a day to boost Vitamin D production. Through regular sun exposure (10-15 minutes a day), humans can make their own Vitamin D in the form of D3. People should use caution, however, as sunscreen blocks the production of Vitamin D (since the skin does not get direct exposure to ultraviolet B light).
Sun exposure can cause skin care and other issues, so moderation is key. Exposure to sunlight for 20-30 minutes is estimated to result int he production of approximately 10,000 IU of vitamin D, although this will vary based on location, heritage, and other factors. Sunlight is the easiest method to acquire vitamin D; by contrast, a person would have to drink roughly 25 glasses of 8oz milk to get the same 10,000 IU of vitamin D.
According to Dr. Jacqueline Chan, increasing the surface of the skin exposed to the sun proportionately decreases the amount of time needed in the sun to produce the same amount of vitamin D. The duration of the sun exposure should be no more than about half the amount of time it takes for the skin to turn pink ( 38 ). Dr. Chan also says that in order to make vitamin D, "The sun must shine directly on skin without being blocked by sunscreen, glass and most plastics. Glass and most plastics block UVB, the part of the spectrum that converts pro-vitamin D3 but allow passage of UVA which contributes to skin cancer." ( 38 )