Mitochondria, by virtue of their biochemical functions, are a natural candidate as a direct target for the calorigenic effects of thyroid hormones. Going further, we can see that mitochondria are highly dependent on thyroid hormones (thus iodine) for their very existence. Thyroid hormones are like the “signal” to make more mitochondria. Thyroid hormone (T3) has a profound effect on mitochondrial biogenesis; without T3, there will be less or no mitochondria. On the other hand, if mitochondria are damaged or depleted due to some reason other than too little T3, then existing T3 has “nothing to act on.” You can have all the T3 in the world, but without mitochondria, there will not be any energy. Again, you can see the circular downward spiral of both host cell and mitochondria that can occur if either 1) too little or no T3 exists, or 2) too little or no mitochondria exist.