The Dose OF Vitamin D3

One gram is 40,000,000 (40x106) IU, equivalently 1 IU is 0.025 µg.

Recommendations are: 15 µg/d (600 IU per day) for all individuals (males, female, pregnant/lactating women) under the age of 70 years-old. For all individuals older than 70 years, 20 µg/d (800 IU per day) is recommended. A growing body of researchers question whether the current recommended adequate levels are sufficient to meet physiological needs, particularly for individuals deprived of regular sun exposure or those at higher risk such as those with higher-melanin content in the skin (i.e. those whose ancestors are African, Middle Eastern, Latin American, or Asian), the obese, and those who live far from the equator. The upper limit (UL) for vitamin D has been recommended as 4,000 IU daily. The 4,000 IU cut-off was determined by the Institute of Medicine in 2010 after reviewing the then-current medical literature, finding that toxicity had consistently occurred when doses of 40,000 IU daily were taken, and that there was a single case of toxicity above 10,000 IU daily; this case of toxicity occurred under circumstances which have led other researchers to dispute it as a credible case to consider when making vitamin D intake recommendations. The Institute of Medicine did not find evidence of toxicity between 4,000 IU and 10,000 IU, so the 4,000 IU number is more of an estimate than a number based on evidence of toxicity above 4,000 IU. Patients with severe vitamin D deficiency will require treatment with a loading dose, its magnitude can be calculated based on the actual serum 25-hydroxy-vitamin D level and body weight. However, there also exists a hypothesis which disadvises from supplementing vitamin D if not really necessary: Vitamin D might support the emergence of allergies and adulterate already existing allergies and autoimmune diseases. But that is merely an unproved hypothesis.

Also, there is a therapy for rickets utilizing a single dose, called stoss therapy in Europe - taking from 300,000 IU (7,500 µg) to 500,000 IU (12,500 µg = 12.5 mg), as a single dose, or two to four divided doses.
The 25-hydroxy vitamin D (calcifediol) blood test is used to determine how much vitamin D is in the body. The normal range of calcifediol is 30.0 to 74.0 ng/mL.
"Vitamin D toxicity can result from regular excess intake of this vitamin, and may lead to hypercalcemia and excess bone loss. Individuals at particular risk include those with hyperparathyroidism, kidney disease, sarcoidosis, tuberculosis, or histoplasmosis. Chronic hypercalcemia may lead to serious or even life-threatening complications, and should be managed by a physician. Early symptoms of hypercalcemia may include nausea, vomiting, and anorexia (appetite/weight loss), followed by polyuria (excess urination), polydipsia (excess thirst), weakness, fatigue, somnolence, headache, dry mouth, metallic taste, vertigo, tinnitus (ear ringing), and ataxia (unsteadiness). Kidney function may become impaired, and metastatic calcifications (calcium deposition in organs throughout the body) may occur, particularly affecting the kidneys. Treatment involves stopping the intake of vitamin D or calcium, and lowering the calcium levels under strict medical supervision, with frequent monitoring of calcium levels. Acidification of urine and corticosteroids may be necessary."
There are conflicting reports concerning the absorption of cholecalciferol (D3) versus ergocalciferol (D2), with some studies suggesting less efficacy of D2, and others showing no difference. At present, D2 and D3 doses are frequently considered interchangeable, but more research is needed to clarify this.

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