Adrenal / Hypo...endocrino

Your adrenal glands are the last stage in the "Hypothalamic–pituitary–adrenal axis" (see Wikipedia). So your adrenals/thyroid can be perfectly ok but still not deliver any hormones because they have not be prompted to do so because of pituitary disease or a hypothalmus problem.

A '9 am cortisol [blood] test' (which is the high point of your daily cycle) will determine if your adrenals [and thus the axis] are working normally - I suspect ihat your endo expects it will be low. This is the baseline.

The ACTH test [again, see Wikipedia] is to simulate the signal hormone that your pituitary SHOULD be sending to your adrenals, telling them to produce cortisol. If there is no change, the problem is your adrenals which suggests Addison's. [You said that she has almost ruled out Addison's but she is right to order a cheap and simple blood test to firmly rule it out]. On the other hand if there is a [normal] adrenal response, then your adrenals are fine and the problem is upstream - far more likely to be your pituitary than your hypothalmus,

If the tests suggest the pituitary then the next stage will be an MRI scan to see what is going on.

If you do need an op on your pituitary, don't read anything about the op that is older than five years because nowadays it really is a minimally invasive and painless procedure.

Finally, levothyroxine is a synthetic copy of the natural thyroxine that your thyroid should produce naturally - but only as much and as little as your pituitary tells to. A lazy pituitary means an inactive thyroid. Which is why healing the cause of hypoadrenal heals the cause of hypothyroid.

I started out with an overactive thyroid.  Radio Active treatment changed that - now an underactive thyroid. Ruined for life.  Never was able to function as well as other people.  Till a nature path figured out that I have adrenal problems.  (saliva test)  Now I'm taking Cortex and Dessicated thyroid meds and have improved a lot ever since.  

Perhaps this sheds some light on your problem.  

Knowlege of hormones and endocrinology has come on in leaps and bounds and is still being discovered. Radiation treatment for hyperthyroidism means you had a tumour that would have killed you if left untreated. But inevitably your thyroid was damaged too. Nowadays you would be prescribed levothyroxine to replace the missing natural thyroxine.

Please don't be taken in by quacks ['naturopaths', worse still 'homeopaths'] who have a little knowlege, no humility, and a lot of exploitation. But equally, don't expect your GP to have the same detailed knowledge as a specialist - for example, an average GP will see just one case of pituitary disease in a career. Push for reference to an endocrinologist - it is just unfortunate that loony right ideology has forced GPs into rationing treatments..