A 28-year-old woman with a history of regular menstrual cycles presents with secondary amenorrhea for 3 months. She denies pregnancy, weight loss, or stress. Examination reveals galactorrhea. TSH is normal. What is the most appropriate next investigation?

Explanation

## Clinical Scenario Analysis This patient presents with: - **Secondary amenorrhea** (3 months; previously regular cycles) - **Galactorrhea** (pathognomonic sign of hyperprolactinemia) - **Normal TSH** (excludes hypothyroidism, a common cause of elevated prolactin) - No obvious precipitants (no pregnancy, weight loss, stress) The combination of amenorrhea + galactorrhea strongly suggests **hyperprolactinemia**, most commonly due to a **prolactin-secreting pituitary adenoma (prolactinoma)**. ## Diagnostic Algorithm for Amenorrhea with Galactorrhea ```mermaid flowchart TD A[Amenorrhea + Galactorrhea]:::outcome --> B[Measure serum prolactin]:::action B --> C{Prolactin elevated?}:::decision C -->|Yes| D[Prolactin > 200 ng/mL?]:::decision C -->|No| E[Other cause: TSH, FSH/LH]:::action D -->|Yes| F[Pituitary MRI]:::action D -->|No| G[Repeat prolactin + MRI if high suspicion]:::action F --> H[Prolactinoma confirmed]:::outcome ``` ## Why Serum Prolactin Is Correct **Key Point:** Serum prolactin measurement is the **first and most specific investigation** for confirming hyperprolactinemia in a patient with amenorrhea and galactorrhea. **High-Yield:** Prolactin should be measured: 1. **Fasting** (morning, 8–10 AM) — prolactin is pulsatile and elevated by stress, exercise, sleep 2. **At least 3 hours after waking** — to avoid stress-induced elevation 3. **Before any other investigations** — to establish the diagnosis **Normal prolactin:** < 25 ng/mL (< 500 mIU/L) **Pathological elevation:** > 25 ng/mL **Highly suggestive of prolactinoma:** > 200 ng/mL **Clinical Pearl:** A prolactin level > 200 ng/mL has ~95% specificity for prolactinoma. Levels 25–200 ng/mL require MRI to exclude microadenoma or other causes (medications, hypothyroidism, pregnancy, chest wall irritation). ## Investigation Sequence | Step | Investigation | Purpose | |---|---|---| | 1 | **Serum prolactin** | Confirm hyperprolactinemia | | 2 | TSH (if not already done) | Exclude hypothyroidism | | 3 | **Pituitary MRI** | Visualize adenoma if prolactin elevated | | 4 | Visual field testing | Assess mass effect (if macroadenoma) | **Mnemonic: CHOPS — Causes of Hyperprolactinemia** - **C**hemical (dopamine antagonists: metoclopramide, antipsychotics, antidepressants) - **H**ypothyroidism (TRH stimulates prolactin) - **O**piate use - **P**ituitary adenoma (prolactinoma) — most common pathological cause - **S**tress, sleep, seizures, chest wall irritation ## Why Other Options Are Incorrect **Pituitary MRI (Option 2):** While MRI is essential to visualize a prolactinoma, it is **not the first investigation**. MRI should be ordered **after prolactin is confirmed to be elevated**. Ordering MRI without confirming hyperprolactinemia is inefficient and costly. **Progesterone challenge test (Option 3):** This test assesses endogenous estrogen status and is used to differentiate anovulation from outflow obstruction. It is **not indicated** when galactorrhea is present, as galactorrhea indicates hyperprolactinemia, not a primary ovulatory disorder. **Estradiol and FSH levels (Option 4):** These assess the hypothalamic–pituitary–ovarian axis. While they may be useful in other causes of amenorrhea (ovarian failure, hypogonadotropic hypogonadism), they are **not the first-line investigation** when galactorrhea is present, as galactorrhea is a specific marker of hyperprolactinemia.