## **Core Concept**
Galactorrhoea refers to the spontaneous flow of milk from the breast not associated with childbirth or nursing. It is primarily caused by an imbalance of hormones that regulate lactation, particularly an excess of prolactin, which stimulates milk production.

## **Why the Correct Answer is Right**
The correct answer, **Prolactin**, is right because prolactin is the hormone directly responsible for lactation. Elevated levels of prolactin, a condition known as hyperprolactinemia, can lead to galactorrhoea. Prolactin acts on the mammary glands to stimulate milk production. When prolactin levels are high, it can cause milk production and secretion, even in non-pregnant women or in men.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain **Psychiatric drugs** can cause hyperprolactinemia as a side effect, they are not the direct cause of galactorrhoea but rather a potential contributor through their effect on prolactin levels.
- **Option B:** **Estrogen** influences breast development and preparation for lactation but does not directly cause milk secretion. Estrogen levels increase during pregnancy and help in the growth of the breast tissue, but it's prolactin that directly stimulates milk production.
- **Option C:** **Thyroid hormone** imbalances can affect menstrual cycles and have various systemic effects, but they are not a direct cause of galactorrhoea. However, hypothyroidism can indirectly lead to hyperprolactinemia.
- **Option D:** This option is not provided, but based on the correct answer being prolactin, any other option not directly linked to prolactin or its action would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that galactorrhoea can be a sign of a prolactinoma, a type of pituitary adenoma that secretes excess prolactin. Therefore, patients presenting with galactorrhoea should be evaluated for hyperprolactinemia and potentially for a pituitary tumor.

## **Correct Answer:** C. Prolactin.