There are so many things which cause a body to dial back on its thyroid production and T4-to-T3 conversion! Malnutrition (ie inadequate raw materials), illness, infection, excessive omega-6 or insufficient saturated fat, stress, a very-low-calorie diet, too-heavy exercising.... Anything which makes the body think it's in danger of hard times coming and needs to garner its resources -- a siege situation -- is enough to make it slow metabolism via the thyroid.
As i read about the assaults that "tainted" foodstuffs make (my histamine studies), and the postulated infective origins of Chronic Fatigue Syndrome (CFS/ME), i can't help but connect dots until i get back to thyroid function. (It may not be my only tool, but i DO have a hammer....) I had already supposed that there was a chicken/egg situation going on before, and this complicates the matter further.
Bad nutrient absorption (amino acids, vitamin A, iron or B12 perhaps) because one has poor stomach-acid production due to low thyroid will lower thyroid function which will lower stomach acid which will ... ad infinitum. Or h. pylori infection can lower stomach acid which will also .... Or other viral, bacterial or mycoplasma issues -- whew. All roads lead to Rome.
So for those of us who suffer from thyroid misfunction (low, high or fluctuating), does the trouble originate in a faulty body, or do infective or nutritional influences cause the poor function which snowballs into worse function? I suspect all the above.