From Wikipedia (2017):
Cholecalciferol, also known as vitamin D3 and colecalciferol, is a type of vitamin D found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin D deficiency including rickets. It is also used for familial hypophosphatemia, hypoparathyroidism that is causing low blood calcium, and Fanconi syndrome. It is taken by mouth. Vit D3 is a secosteroid, that is, a steroid molecule with one ring open. Cholecalciferol is inactive: it is converted to its active form by two hydroxylations: the first in the liver, the second in the kidney, to form calcitriol, whose action is mediated by the vitamin D receptor, a nuclear receptor which regulates the synthesis of hundreds of enzymes and is present in virtually every cell in the body. After being converted into 1,25-dihydroxyvitamin D, it works by increasing the uptake of calcium by the intestines. Food in which it is found include some fish, cheese, and eggs.
Vitamin A and D deficiencies associated with incident tuberculosis in HIV-infected patients initiating antiretroviral therapy in multinational case-cohort study. Tenforde MW, Yadav A, Dowdy DW, Gupte N, Shivakoti R, Yang WT, Mwelase N, Kanyama C, Pillay S, Samaneka W, Santos B, Poongulali S, Tripathy S, Riviere C, Berendes S, Lama JR, Cardoso SW, Sugandhavesa P, Christian P, Semba RD, Campbell TB, Gupta A; NWCS319 and ACTG 5175 study team. J Acquir Immune Defic Syndr. 2017 Feb 6