Take this to your doctor. If competent she will test you for hypothyroidism (and all other common causes of fatigue). Your test will show that your blood hormone levels are normal. At this point, you have a mysterious unexplained syndrome in which the primary symptom is chronic fatigue, but which overall shows similarities to hypothyroidism. You have Chronic Fatigue Syndrome. Suppose that the symptom that bothers you most is widespread pain. Then you will eventually be diagnosed with Fibromyalgia. Should you complain mostly about alternating constipation and diarrhoea, then you have Irritable Bowel Syndrome.

Or you have a psychiatric, inflammatory, genetic or other cause of fatigue, pain or bowel disturbance. Yes, undiagnosed thyroid issues could cause this but why is this more likely than other possibilities?

We should see that these syndromes have exploded in prevalence since 1970, when diagnosis of endocrine disorder by clinical symptoms went out of fashion in favour of diagnosis by blood hormone level tests.

This is weak evidence, where strong or at least more numerous arguments are needed.

In particular, it is known that the principal characteristic of Chronic Fatigue Syndrome is Mitochondrial Dysfunction [4] . I contend that this is principally caused by lack of the hormone T3 in cells, for reason or reasons currently unclear.

Why? Why not an alternative cause, such as a combination of thousands of genes, mostly unrelated to the thyroid?

I believe that that is exactly what we see. They are known as the 'somatoform' disorders, because they are thought to be all in the mind. By those who have never had one.

Not true. Plenty of people have and diagnose somatoform disorders.

But I am tempted also to include other mysterious diseases without known causes and with symptoms plausibly explained by endocrine hormone abnormalities, such as Bipolar Disorder, Depression, and the 'Metabolic Syndrome', which may do exactly what it says on the tin.

But plenty is already known about the pathophysiology of bipolar disorder, depression and so on. e.g. depression seems likely to be caused by many thousands of genes (either that or some unidentified environmental stimulus, though we tend to have looked quote closely for those) (1)

Why not learn about the conditions and the evidence about their causes and then argue forward to some hypothesis rather than priveleging your thyroid-based hypothesis and trying to apply it to all available diseases?

At least an experiment is proposed here, but if you wanted to actually implement it, you would need a placebo control design, and you would approach disease interest groups or elsewhere for participants.

1) http://www.ncbi.nlm.nih.gov/pubmed/22472876