**Core Concept**
Prolactin inhibition during pregnancy is crucial for preventing premature milk production. This phenomenon is mediated by a hormone that suppresses prolactin release from the pituitary gland.

**Why the Correct Answer is Right**
The hormone responsible for inhibiting lactation during pregnancy is **Prolactin Inhibiting Factor (PIF)**, also known as **Dopamine**. Dopamine, produced by the hypothalamus, inhibits the release of prolactin from the anterior pituitary gland through D2 receptors. This ensures that milk production is delayed until after childbirth. The dopamine hypothesis of prolactin regulation is well established, with dopamine receptors playing a crucial role in modulating prolactin secretion.

**Why Each Wrong Option is Incorrect**
**Option A:** **Estrogen** - While estrogen levels do increase during pregnancy, they do not directly inhibit lactation. Instead, estrogen stimulates the growth of the breast tissue, preparing it for milk production after childbirth.
**Option B:** **Progesterone** - Progesterone levels also rise during pregnancy, but it does not directly inhibit lactation. Progesterone maintains pregnancy by promoting implantation and preventing uterine contractions.
**Option C:** **Thyroid-stimulating hormone (TSH)** - TSH is a hormone produced by the pituitary gland that stimulates the thyroid gland to produce thyroid hormones. It does not play a role in lactation or prolactin regulation.

**Clinical Pearl / High-Yield Fact**
In cases of hyperprolactinemia or galactorrhea, medications that block dopamine receptors (e.g., metoclopramide) can exacerbate the condition, while dopamine agonists (e.g., bromocriptine) can reduce prolactin levels and alleviate symptoms.

**Correct Answer:** D. Dopamine.