Hi Bronagh.
Sorry to hear about MaiTai, Triaditis is what the vets think my Cora has (I'm the other lady HelenD was talking about), but it is very difficult to diagnose, so I'm surprised your vet has come up with that so quickly. It is basically a liver, pancreas and cholangiohepatitis problem, and the trick is, as someone else said, balancing the meds.
If the liver is affected, the immune system will be low, so that's what the antibiotics are for.
It is horrible and fast, Cora collapsed with it with no warning, and was only given days to live, but that was 15 months ago. The day I collected her from the vet she was like a wet dishcloth.
There are two liver supplements, Hepatosyl and one starting with an L, not sure of it's name. Food-wise, I give Cora the Royal Canin Digest Sensitive and anything else or sensitive tums, same as Helen (thanks Helen), and try to keep Cora's diet as additive free as I can. If you can afford it, go organic.
Good Luck, it does take time with this, if MaiTai is a younger cat, then I think the chances of recover will probably be better.