Which of the following hormones is an example of a peptide hormone?
**Core Concept**
Peptide hormones are a type of hormone that consist of short chains of amino acids, which are linked together to form a polypeptide chain. These hormones are produced by endocrine glands and play a crucial role in various physiological processes, including growth, metabolism, and reproduction.
**Why the Correct Answer is Right**
Peptide hormones work by binding to specific receptors on the surface of target cells, triggering a cascade of intracellular signaling pathways that ultimately lead to a specific response. Examples of peptide hormones include insulin, growth hormone, and oxytocin, which are all produced by endocrine glands such as the pancreas, pituitary gland, and hypothalamus. The binding of these hormones to their receptors activates enzymes such as adenylyl cyclase, which increases the production of cyclic AMP (cAMP), a secondary messenger molecule that triggers various cellular responses.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, although it is a hormone, it is a steroid hormone, not a peptide hormone. Steroid hormones are derived from cholesterol and are lipophilic, allowing them to diffuse through cell membranes to bind to specific receptors.
**Option B:** This option is incorrect because, although it is a hormone, it is a catecholamine, not a peptide hormone. Catecholamines, such as adrenaline (epinephrine) and noradrenaline (norepinephrine), are derived from the amino acid tyrosine and are hydrophilic, requiring transport across cell membranes to bind to specific receptors.
**Option C:** This option is incorrect because, although it is a hormone, it is not a peptide hormone. Thyroid hormones, such as triiodothyronine (T3) and thyroxine (T4), are derived from the amino acid tyrosine and are lipophilic, allowing them to diffuse through cell membranes to bind to specific receptors.
**Clinical Pearl / High-Yield Fact**
Peptide hormones are often regulated by negative feedback mechanisms, where the production of a hormone is inhibited by the presence of high levels of the hormone itself or its metabolites. This helps to maintain homeostasis and prevent excessive hormone production.
**Correct Answer:** C. Growth hormone is an example of a peptide hormone.