
Have no personal experience with hyperthyroidism, but the article JG has linked to mentions the important thing is a high quality diet mainly made up from meat, and when Googling I found other articles that also mentioned raw should be fine. Such as this:
http://www.onlynaturalpet.com/knowledgebase/knowledgebasedetail.aspx?articleid=74 I really would not cook the food. Having no teeth is no problem at all -either feed minces (PLENTY of bones ground into the Prize Choice chicken and lamb mince for instance -but beware, with too much bones you will get severe constipation so I found it is vital to also offer beef mince as that has the opposite effect, so I mix beef and chicken, then give some meals of lamb, some of chicken with liver etc) or use one of the compete raw diets that is minced blocks or nuggets (Natural Instinct or Natures Menu both do ones -not cheap though, but for ONE cat should be doable). I've not yet come across a cat that will refuse the minces if offered nothing else, but for the really picky eater, far better than dry food or average cans are the Natures Menu pouches that are complete and with a very high meat content. Or Applaws cans or pouches, as they are of up to 99 % pure meat.