On amazon - you can get the BetterYou DLUX vitamin d3 oral spray. You just spray it into your mouth - minty taste - it is better absorbed into the bloodstream then taking the pills/softgels, as they are passed through the ‘gut’ and most is not utilised. The spray is only 3,000ius - but as 98% is absorbed it is better. They also do one for infants - fruit flavoured.
For testing - l prefer to use the CityAssays Lab B/ham - it is a postal service - just a pinprick test. And is much more comprehensive then doing it at the GP’s. l think it now costs £28. You need to get your levels up to 150/225 nmols.
Don’t forget to keep up with all the B vits - especially B12. As for some unknown reason - we with MS - do not absorb these vits enough and need high doses.