The most likely cause is Low Cortisol Explanation: 90% of all hypothyroid conditions are Hashimoto's. This is an autoimmune (your body attacks itself) condition. With this condition, the thyroid has antibodies that Parallel investigation of oxidative stress and nutritional profile in women with Hashimoto's thyroiditis Clinical Nutrition Hashimoto's Thyroiditis: The Destructive Thyroid Disease That Slows You Down The Wellness Way Shop Hashimoto's Thyroiditis Supplements Lower Antibodies Naturally Frontiers Association between fatigue, peripheral serotonin, and L carnitine in hypothyroidism and in chronic fatigue syndrome