Final answer:
The most abundant thyroid hormone in the circulation is T4 (thyroxine), which makes up about 90% of circulating thyroid hormone. T3 (triiodothyronine) makes up only about 10% and is more potent than T4.
Step-by-step explanation:
The most abundant thyroid hormone in the circulation is T4 (thyroxine). T4 makes up about 90% of circulating thyroid hormone, while T3 (triiodothyronine) makes up only about 10%. However, most of the T4 is converted to T3 by target tissues. It is important to note that T3 is more potent than T4 and is responsible for most of the effects of thyroid hormones.