Could be that he just was misdiagnosed? Peat talks about this all the time. Someone goes to the endocrinologist with a skinny body, in the docs mind it can ONLY be hyperthyroidism, but it's actually highly unlikely. All his symptoms could be a result of compensatory mechanisms (stress hormones) to hypothyroidism.
Yeah I have heard this too, but in my friends case I doubt it. She has a hereditary condition which her mother also had, and felt much better when she took some thyroid lowering drug. She also lost so much weight when it occurred, it seems more in line with increased thyroid hormones to me. But of course I cant be sure 100%. Charlie's explanation about burning through all the cofactors seems to make sense though, and TheCalciumCad's experience also seems to fit with that.