Hi cherrypoker,
I don't know anything about Rocky, but by law vets can only continue repeat prescriptions for a number of months before the animal has to be seen again.
In many cases the vet won't prescribe more drugs without seeing the animal as the drug levels need to be monitored closely to judge their effect. Such drugs include ones for hormonal diseases, like Cushing's, Addison's and hypo/hyperthyroidism, where the amount of drug required changes with time.
Form your responses, does Rocky have goitre causing low levels of thyroid hormone? If so, your vet needs to see him again for the above reasons. The amount of drug Rocky needs may gradually increase, and the best judge of that is your vet. The more often he sees Rocky, the better he will be able to see how well the tablets are working and monitor changes in his condition.