A 32-year-old woman with a 5-year history of schizophrenia on olanzapine 10 mg daily presents with amenorrhea for 8 months, galactorrhea, and reduced libido. Her symptoms are otherwise well-controlled. Which investigation is most appropriate to evaluate the cause of her endocrine symptoms?

Explanation

## Antipsychotic-Induced Hyperprolactinemia **Key Point:** Antipsychotics, especially typical agents and risperidone/paliperidone, cause hyperprolactinemia by blocking dopamine D2 receptors in the tuberoinfundibular pathway. Serum prolactin measurement is the first-line investigation. ### Clinical Context: Olanzapine and Prolactin Olanzapine is a second-generation antipsychotic with **moderate prolactin-elevating potential** (less than risperidone but more than aripiprazole or quetiapine). The patient's triad of amenorrhea, galactorrhea, and reduced libido is classic for **drug-induced hyperprolactinemia**. ### Why Serum Prolactin Is the Correct Investigation 1. **Direct measurement** — confirms the suspected diagnosis of hyperprolactinemia. 2. **Baseline for comparison** — establishes the degree of elevation and guides management decisions (dose reduction vs. switching agent vs. dopamine agonist). 3. **Non-invasive and cost-effective** — simple blood test, widely available. 4. **Guides next step** — if prolactin is markedly elevated (> 200 ng/mL), MRI pituitary may be warranted to exclude prolactinoma. **High-Yield:** Serum prolactin > 200 ng/mL or a rapid rise warrants MRI pituitary to exclude a prolactinoma, which can occur independently of antipsychotic use. ### Mechanism of Antipsychotic-Induced Hyperprolactinemia ```mermaid flowchart TD A[Antipsychotic blocks D2 receptors]:::action --> B[Loss of dopamine inhibition of prolactin]:::action B --> C[↑ Prolactin release from lactotroph cells]:::outcome C --> D[Amenorrhea, galactorrhea, sexual dysfunction]:::outcome D --> E{Prolactin level?}:::decision E -->|Mildly elevated| F[Consider dose reduction or switch]:::action E -->|Markedly elevated >200| G[MRI pituitary to exclude prolactinoma]:::action ``` ### Investigation Algorithm for Antipsychotic-Induced Endocrine Symptoms | Step | Investigation | Rationale | |---|---|---| | 1 | Serum prolactin | Confirm hyperprolactinemia | | 2 | If prolactin > 200 ng/mL | MRI pituitary with contrast | | 3 | If prolactin mildly elevated | Trial of dose reduction or agent switch | | 4 | Thyroid function tests | Exclude concurrent hypothyroidism | ### Why Other Options Are Incorrect **MRI pituitary** is a second-line investigation, performed only if: - Prolactin is markedly elevated (> 200 ng/mL) - Rapid rise in prolactin despite stable antipsychotic dose - Clinical suspicion of pituitary adenoma (severe headache, visual field defect) In this case, the temporal relationship to olanzapine initiation makes drug-induced hyperprolactinemia the most likely diagnosis. **Thyroid function tests** may be useful as part of comprehensive metabolic screening but do not directly address the endocrine symptoms (amenorrhea, galactorrhea are not typical of thyroid dysfunction). **Pelvic ultrasound** may assess ovarian morphology but does not identify the cause of amenorrhea (hyperprolactinemia). **Clinical Pearl:** Aripiprazole and quetiapine have minimal prolactin-elevating effects and are preferred agents in patients with antipsychotic-induced hyperprolactinemia. Dopamine agonists (bromocriptine, cabergoline) can be added if dose reduction or switching is not feasible. [cite:Harrison 21e Ch 385; Kaplan & Sadock 21e Ch 29]