Hi Cataholic

From what you have described it actually sounds like Tex has hyperthyroidism (too much) and not hypothyroidism (too little). The thyroid gland produces thyroid hormone which regulates metabolism (amongst other things). This is a condition that can develop later in life (in both humans and animals), so the Vet may not have caught it before because it wasn't there before. But that still doesn't explain the constant vomiting