Simon, failure of the hypothalamus and pituitary is known about, and referred to as 'central hypothyroidism', and it does indeed produce the same symptoms but normal TSH values. However you can detect it by other blood tests.

But it's very rare. So rare that it's not cost-effective to test for it unless you've got some reason to look. Some undiagnosed cases will end up getting labelled as CFS, and that's a reason to test other hormone levels too if you can't explain someone's problems, but it's not nearly common enough to explain the whole thing (assuming very minimal levels of competence in those investigating).