Just look up any chemical database like Kegg or Reactome. Vitamin D reactions tend to consume magnesium, boron, etc. Supplementing those is very important, both for effectiveness and long-term safety.
I know some multiple sclerosis patients that are taking megadoses, under medical supervision. Doctors have learned that cofactors are crucial to avoid nasty side effects.
Have you tried taking Iodine?
I have Hashimoto's Disease.
Thing is, many people with genes for it never develop the disease, or develop it only later in their life... I started showing symptons when I was 14.
Completely by coincidence I found out it was the government's fault, a lot of people, me included, developed the disease suddenly, at same time, and what happened at that time was that the government increased mandatory iodine levels in the salt, it caused an explosion of hypothyroidism cases, they then scaled it back and new cases returned to the average.
Problem is that people that had the disease triggered by Iodine never come back to normal no matter what.
If you take 7000 IU it might take years for you to slowly develop vitaminosis. Get your blood checked regularly to keep track.
Vitamin D is fat-soluable and not flushed out by the kidneys. It sticks in the body fat and therefore accumulates.
Bodily regulation typically happens before synthesis after sunlight exposure but if you take already-complete vitamin D, a hormone, then no regulation can keep a check anymore as you have effectivly bypassed it.
7000 IU might be fine for a while but I'd definitely keep track of the values and if there is medical evidence that higher is necessary for hypothyroidism, keep kidneys and other values in check.