Final answer:
Triiodothyronine (T3) is the primary thyroid hormone responsible for the observed effects of the thyroid hormone.
Step-by-step explanation:
The primary thyroid hormone responsible for the observed effects of the thyroid hormone is triiodothyronine (T3). Although thyroxine (T4) makes up about 90% of circulating thyroid hormone, T3 is much more powerful and is responsible for most of the effects of thyroid hormones. Target tissues in the body convert T4 to T3, and T3 is the active form that binds to receptors on the mitochondria, increasing ATP production and regulating gene expression to control metabolism.