A company called Holtorf Medical Group writes in Are All Chronic Fatigue Syndrome and
Fibromyalgia Patients Low Thyroid:
There is mounting evidence that there is low thyroid activity present in the majority of chronic fatigue syndrome and fibromyalgia patients. [...] Studies demonstrate that in addition to an increased incidence of primary hypothyroidism in CFS, there is a combination of secondary, tertiary and thyroid resistance in the overwhelming majority of CFS patients, despite having normal thyroid tests.
On the other hand the Mayo clinic describes Chronic fatigue syndrome by saying:
Chronic fatigue syndrome is a complicated disorder characterized by extreme fatigue that can't be explained by any underlying medical condition. The fatigue may worsen with physical or mental activity, but doesn't improve with rest.
The Mayo clinic also doesn't say anything about thyroid activity.
How good is the case that chronic fatigue syndrome is often caused by low thyroid activity?