The diagram above illustrates feedback control as exerted by the hormone thyroxine. Following surgical removal of the thyroid gland, the level of TSH in the blood will increase. Which of the following best explains this increase?

The first three options may be true in the sense that thyroxine remains in the bloodstream after surgery. However, thyroxine inhibits, or reduces TSH secretion by inhibiting the anterior pituitary, which synthesizes TSH. Accordingly, a reduction in thyroxine will result in reduced inhibition of the anterior pituitary, resulting in normal or enhanced TSH production. TSH would normally be consumed in the thyroid gland, but due to the gland's removal TSH would instead accumulate in the bloodstream.