We changed our cats' diet in the Spring of 2010, hoping that the ones who needed to lose some weight would do so slowly and gradually, as is safe for cats. To our delight, most of them did, including Molly. She was quite pudgy to begin with, so we were pleased with her progress. She even continued to lose well after everyone was stabilized on the new diet. We weren't alarmed by this as the weight loss was slow and she certainly was not underweight.
When we took her in to the vet for a routine exam and bloodwork in the Fall of 2011, we were a bit startled to find that she has hyperthyroidism. That's why she was doing so well on the diet: she was "cheating!" Fortunately, she exhibited no other signs of the disease apart from the weight loss. 
She is the second of our cats to be diagnosed with this illness. Tamerlane was diagnosed with it in 2005. Our plan for him was to stabilize him on oral medication and save up money for radioactive iodine treatment; however, he was unable to tolerate the medication at all. We went straight for the radioactive iodine and he's not had any problems since then.

Molly, in contrast, is doing spectacularly well on oral Methimazole. We will eventually pursue radioactive iodine treatment for her also, but there's no rush.