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“Burn out” refers to the condition related to the level of energy a person has during the day. Typically associated with hypothyroidism, this symptom is caused by low thyroxine (T4) and triiodothyronine (T3) levels in circulation, with T3 being more metabolically active in the cell.
If a patient had the thyroid removed surgically or ablated by radioiodine, and is subsisting entirely on levothyroxine (LT4) tablets as replacement, an enzyme called deiodinase produced by the body is needed to convert the T4 into T3. If this enzyme is deficient or absent, T4 cannot be converted to T3 and the patient experiences “burn out.” Moreover, unsuspected illnesses and fasting result in a decrease of T3 production that may contribute to fatigability, lethargy, sleepiness and cold intolerance.