This is my own personal take.  With a 15 yr. old cat who's doing well on the
Tapazole/methimazole tablets, & is not a problem pilling, I'd stick w/ the
pills.

If she was a few years younger, or if the pills were pricey, or if she
*hated* being pilled, or if she wasn't tolerating the pills well, etc. -
*then* I'd consider the radiation treatment.

Cathy

--
"Staccato signals of constant information..."
("The Boy in the Bubble")  Paul Simon

I don't think I'd opt for radioiodine tx for a 15 year old.   The stress of
5-7 days hospitalization may have deleterious effects, but mostly because
there's >30% chance that her hyperthyroidism is masking underlying chronic
renal failure.  If she does have, or develops CRF, her medication can be
adjusted so that you can strike a balance between an "acceptable" level of
hyperthyroidism and an "acceptable" level of azotemia - -- Radioiodine tx is
*irreversible* and nonadjustible.

If she develops adverse effects from methimazole, you can easily switch her
to carbimazole which produces fewer and milder adverse effects.  Carbimazole
is converted to methimazole in vivo. Aside from this, there are no
significant pharmacokinetic differences between the two drugs. The only
disadvantage associated with carbimazole is that its only available through
compounding pharmacies in the US.

If she's not a problem to pill, I wouldn't even consider radioiodine tx at
her age.

Best of luck.

Phil.