You would need a lot more research that (I don't believe has been done) on how often you really need to adjust your thyroid. As cdrugde mentions, thyroid testing is done infrequently - on the order of months. To have an implantable system that fired off that infrequently is probably not really worth it. Dogma is that thyroid hormones don't change that much - at least a clinically noticeable values.
Most of the thyroid replacement research these days seems to be around the issue of 'minor' thyroid hormones, mostly T3. Even with this issue, we don't have good data. And T3 is produced at about 10% of the rate of T4 (the traditional thyroid hormone) so it isn't some scarce little molecule.
But if you really look closely at how much we know about things like this, it's pretty disappointing. This sort of research is slow and very expensive. Given that thyroid pills are pennies and that people do 'pretty well' on that treatment, the impetus to improve things is small.