thyroid-stimulating hormone (tsh) is a long-distance signaling molecule released by the anterior pituitary gland. once released, tsh travels through the bloodstream to the thyroid gland, where it binds to g protein-coupled receptors called tsh receptors. this binding initiates signal transduction pathways that produce two thyroid hormones, t3 and t4. in individuals with hypothyroidism, tsh is released by the pituitary gland normally, but these individuals have consistently low levels of t3 and t4. which of the following best explains why individuals with hypothyroidism have low levels of t3 and t4?