**Core Concept**
Prolactin is a hormone secreted by the pituitary gland that stimulates the growth and secretion of milk by the mammary glands during lactation. It is a key component of the reproductive endocrine system, playing a crucial role in milk production and mammary gland development.

**Why the Correct Answer is Right**
Prolactin binds to its receptor in the mammary gland, triggering a cascade of signaling pathways that stimulate the synthesis and secretion of milk proteins and lipids. The hormone's effects are mediated by the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway, which regulates gene expression and protein production in the mammary gland. This process is essential for the initiation and maintenance of lactation.

**Why Each Wrong Option is Incorrect**
**Option A:** Oxytocin, while involved in milk ejection during lactation, is not responsible for the initiation of milk production. It acts on the myoepithelial cells surrounding the alveoli to induce milk letdown.
**Option B:** Estrogen, a key female sex hormone, is involved in the development of the mammary gland but is not directly responsible for lactation. It plays a role in the differentiation and growth of mammary epithelial cells.
**Option C:** Thyroid-stimulating hormone (TSH), secreted by the pituitary gland, regulates thyroid function and is not involved in lactation. It stimulates the thyroid gland to produce thyroid hormones, which are essential for metabolism and development.

**Clinical Pearl / High-Yield Fact**
Prolactin levels can be elevated in response to stress, nipple stimulation, and certain medications, such as dopamine antagonists. This can lead to galactorrhea, a condition characterized by the spontaneous production of milk in the absence of childbirth or nursing.

**Correct Answer: C. Prolactin.**