T3 inadequacy is usually a systemic phenomenon. It can, therefore, cause energy-deficiency contractures in muscles throughout a patient's body. His contractures are tender to palpation and may wildly refer pain or paresthesia when compressed. Noxious signals from these contractures stimulate the patient's reticular activating system, intensely arousing him. From this point, a complex set of interactions ensue. His arousal causes insomnia and shallow, non-restorative sleep. The sleep disturbance, combined with his ATP deficiency, weakens him and causes sustained fatigue. His ATP deficiency, combined with his arousal, causes constipation, and then spastic colon or irritable bowel syndrome. And the lack of T3 can desensitize the brain's adrenergic receptors, dulling their response to the neurotransmitter norepinephrine. This leaves him depressed.

It's noteworthy that most of the above symptoms are the same as those that get patients the diagnosis of fibromyalgia, or chronic fatigue syndrome.
T3 and Chronic Fatigue