Thyroxin is a hormone that increases metabolic activities within various tissue targets. Low levels of circulating thyroxin

Question

Thyroxin is a hormone that increases metabolic activities within various tissue targets. Low levels of circulating thyroxin trigger the secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary. TSH secretion then stimulates thyroxin production and release by the thyroid gland. The increased level of circulating thyroxin inhibits further secretion of TSH from the anterior pituitary. Based on the information provided, which of the following can most likely be concluded about the TSH-thyroxin loop? 
a. A person taking thyroxin to supplement low thyroxin secretion will produce more TSH. 
b. Increased thyroxin production would cause elevated ribosomal activity in the anterior pituitary. 
c. The structure of the loop would lead to elevated thyroid and tissue activity due to positive feedback. 
d. The feedback mechanism would maintain relatively constant levels of thyroxin throughout tissue targets.

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