a good multivit/multimin is certainly a good idea.
keep in mind that multis are meant to top up healthy people, that are consuming a reasonably healthy diet.
multis are not likely to contain magnesium or zinc in sufficient quantities to balance therapeutic doses of vit d3 and iron. so, it's quite likely you'll need some more therapeutic doses of a few things *in addition to a multi*, given that you've received a diagnosis. but the multi is still a great thing to have in the picture overall.
baseline nutrient testing and then periodic follow-up monitoring over time can really help you select the best route forward.