I just came across this and I'm posting it because it seems to answer the question I've seen so many times about thyroid involvement and CFS/ME. So many people ask if they can feel bad but have normal TSH levels. This answers all of it-and the relationship to chronic illnesses. There is WAY more on the website-it is very much worth reading.....I'm just posting enough to show the basic premise and how to find the page. I wish I had found this years ago https://www.nahypothyroidism.org/deiodinases/#depression Deiodinase type I (D1) D1 converts inactive T4 to active T3 throughout the body, but D1 is not a significant determinant of pituitary T4 to T3 conversion, which is controlled by D2 (1,7,10). D1 but not D2 is suppressed and down-regulated (decreasing T4 to T3 conversion) in response to physiologic and emotional stress (11-22); depression (23-45); dieting (46-51); weight gain and leptin resistance (47-91); insulin resistance, obesity and diabetes (91-99); inflammation from autoimmune disease or systemic illness (11,100,102-115); chronic fatigue syndrome and fibromyalgia (121-125); chronic pain (116-120); and exposure to toxins and plastics (126-134). In the presences of such conditions there are reduced tissue levels of active thyroid in all tissues except the pituitary. The reduced thyroid tissue levels with these conditions is often quoted as a beneficial response that lowers metabolism and thus does not require treatment, but there is no evidence to support such a stance while there is significant evidence demonstrating it is a detrimental response (135-142). In addition, D1 activity is also lower in females (143,144), making women more prone to tissue hypothyroidism, with resultant depression, fatigue, fibromyalgia, chronic fatigue syndrome, and obesity despite having normal TSH levels.