My levels keep going low, no matter how often they top them up, for me I think maybe related to a bile acid malabsorption condition I have (something to do with fat soluable vitamins) and I am not a fan of the sun or bright light (maybe dry eye related) and do tend to keep covered up, curtains closed (although light ones which let light in), wear a hat outdoors, can't tolerate a lot of cow's milk etc...
I tend to be prescribed Hux D3 20000iu Colecalciferol capsules to bounce it up and then look to have it checked after the lighter sunnier months where it can build up more naturally and check if it needs topping up for the Autumn/Winter months.
I've been tested for thyroid a few times before now and they have either checked or I've asked for Vitamin D levels to be checked as usually that which comes back low.