Final answer:
The thyroid hormone synthesized in large quantities is T4, which cells usually convert to T3 for immediate use.
Step-by-step explanation:
The form of thyroid hormone that is synthesized in large quantities is thyroxine, also known as T4, which contains four atoms of iodine. The cells in the body generally convert T4 to the more active form, triiodothyronine or T3, for immediate use rather than storing it. This conversion is crucial because T3 is responsible for most of the effects of thyroid hormones, including increasing the rates of mitochondrial ATP production and metabolism within cells.