Question

Insulin-like growth factor (IGF-1, somatomedin C) is secreted mainly by which organ?

• A. Liver
• B. Pituitary gland
• C. Pancreas
• D. Adrenal glands

Hint:

Growth hormone acts on this organ to make somatomedins (IGF-1).

Answer 1

The Correct Option is A

Step 1: Growth hormone (GH) from the anterior pituitary does not act on bone entirely on its own; many of its growth effects are mediated through intermediary peptides.
Step 2: GH acts on the liver to stimulate production of small proteins called somatomedins. Because their actions resemble insulin, they are termed insulin-like growth factors (IGFs). The most important is somatomedin C, also called IGF-1.
Step 3: So the principal source of circulating IGF-1 is the liver, making option (a) correct. IGF-1 binds a carrier protein in blood, giving it a long half-life of about 20 hours (compared with about 20 minutes for GH), and its blood level follows GH levels.
Step 4: The distractors are wrong: the pituitary secretes GH itself, not IGF-1; the pancreas secretes insulin and glucagon; and the adrenal glands secrete corticosteroids and catecholamines - none of these is the chief source of IGF-1.