What Causes Low Iron Level Despite Taking Iron Tablets?

Hi..recently my husbands blood test was .thanks done and the doc told him that his iron was low so he was given iron tabs.For a month he ate and another test was done.His iron went further done and his thyroxine was little low too.This time the doc told him to continue with iron tabs and also gave him thyroxine tab for a month.Another blood test was done and iron tabs is to continue while thyroxine was stopped.Today another letter came from doc to do another blood test . The doc wants to know the cause of low iron and why iron tabs not working.Hope youwill provide some ans