This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which of the following forms of thyroid hormone is most readily found in the circulation? 
Select and explain. 
A. Tri-iodothyronine (T3)., B. Thyroxine (T4)., C. Thyroglobulin., D. TSH.
B: Thyroxine (T4).
Answer: B: Thyroxine (T4). 
Explanation: Thyroxine and triiodothyronine are formed by the cleavage of thyroglobulin following stimulation by TSH. Most thyroxine is converted to triiodothyronine in liver and kidney. Thyroglobulin is the storage form of thyroid hormone. TSH (thyroid-stimulating hormone) is produced in the anterior pituitary. It is not a hormone secreted from the thyroid but acts to stimulate the synthesis and secretion of thyroid hormones.