Hi Thanks for writing to Health care magic The complications of Hypothyroidism are mostly cardiac in nature. Too vigorous thyroid therapy may precipitate a heart failure. Also adrenal crisis is precipitated by thyroid therapy. Severe hypothyroidism can cause myxedema coma with severe hypoventilation, low body temperature, low sodium levels, hypoxia and hypotension. In all these situations there is a possibility of fainting. Get the thyroid levels checked up. Also get electrolytes, routine blood tests done. A good clinical examination by a specialist will give the cause. Hope this will help u Thanks and regards