In human endocrinology, which of the following hormones is a protein (peptide) hormone rather than a steroid hormone?

Introduction / Context:
Hormones can be classified into different groups based on their chemical structure, such as peptide (protein) hormones and steroid hormones. Knowing which hormones are proteins and which are lipids is important for understanding how they are synthesized, transported in blood, and how they act on target cells. This question asks you to identify which listed hormone is a protein hormone.

Given Data / Assumptions:
- Estrogen is a major female sex hormone involved in reproductive functions.
- Insulin is a hormone involved in regulating blood glucose levels.
- Protein or peptide hormones are built from amino acids, while steroid hormones are derived from cholesterol.

Concept / Approach:
Insulin is a peptide hormone produced by beta cells of the pancreas. It is composed of amino acid chains linked by peptide bonds. Estrogen, on the other hand, is a steroid hormone synthesized from cholesterol in the ovaries and other tissues. Steroid hormones are lipids and are not proteins. Therefore, only insulin among the options is correctly described as a protein or peptide hormone.

Step-by-Step Solution:
Step 1: Remember that peptide and protein hormones are made of amino acids and are generally water soluble. Step 2: Identify insulin as a peptide hormone that helps cells take up glucose from the blood. Step 3: Recall that estrogen belongs to the steroid family, derived from cholesterol, and is lipid soluble. Step 4: Examine the options and find the one that correctly states that insulin is a peptide hormone. Step 5: Select this option as the correct answer, and reject options that misclassify estrogen or both hormones as proteins.

Verification / Alternative check:
Endocrinology references list insulin under peptide hormones and describe its structure in terms of amino acid sequences. Estrogen is consistently categorized with other steroid hormones such as progesterone and testosterone. This classification is based on chemical structure and solubility, confirming that insulin is a protein hormone while estrogen is not.

Why Other Options Are Wrong:
Option A: Estrogen is not a protein hormone; it is a steroid hormone derived from cholesterol and is lipid soluble.
Option C: Both hormones are not protein hormones; only insulin is. Estrogen belongs to the steroid category.
Option D: This option is incorrect because at least one hormone in the list, insulin, is clearly a peptide hormone.

Common Pitfalls:
Students sometimes assume that all hormones are proteins because many are secreted by glands. It is important to remember the distinction between peptide hormones such as insulin and steroid hormones such as estrogen and cortisol. Another pitfall is confusing function with structure; even though both hormones have important regulatory functions, their chemical nature is different.

Final Answer:
The correct answer is insulin, a peptide hormone made of amino acid chains because insulin is a protein hormone, while estrogen is a steroid hormone derived from cholesterol.