I'd been having trouble with hyPOthyroidism due to mercury detox and my doctor (chiropractor into nutrition) just put me on Metagenics Iso D3 ("but only for 6 weeks to help your liver, as then we'll switch you over to Thyrosol which will help your thyroid"). I AM feeling better these days.
Maybe you're on the wrong type of vitamin D???
My chiropractor (57, now competes in marathons, and looks younger than he did over three years ago when I saw him last, so I think he may know a thing or two) has studied vitamins in depth and looked at the statistical results of the products. He saw the consistency of Metagenics brand over others...(like "Standard Process Inc", which is what a lot of health practitioners use, including the one I came from whose products didn't help me as much as I'd hoped).