Which of the following hormones acts on its target tissues by a steroid hormone mechanism of action?

Correct Answer: A

Rationale: Thyroid hormone, an amine, acts on its target tissues by a steroid hormone mechanism, inducing the synthesis of new proteins. The action of antidiuretic hormone (ADH) on the collecting duct (V2 receptors) is mediated by cyclic adenosine monophosphate (cAMP), although the other action of ADH (vascular smooth muscle, V1 receptors) is mediated by inositol 1,4,5-triphosphate (IP3). Parathyroid hormone (PTH), β 1 agonists, and glucagon all act through cAMP mechanisms of action.

Correct Answer: D

Rationale: Amines (e.g., epinephrine, thyroxine) are tyrosine-derived and secreted by thyroid and adrenal medulla.

Correct Answer: B

Rationale: Cyclic AMP is the primary second messenger for hydrophilic hormones (e.g., glucagon).

Correct Answer: B

Rationale: Thyroid hormone feeds back to inhibit TSH release from the anterior pituitary.

Correct Answer: C

Rationale: The pancreas (islets of Langerhans) secretes insulin and glucagon.