A 35-year-old man presents to the endocrinology clinic with complaints of erectile dysfunction and decreased libido for 4 months. He denies penile trauma or relationship issues. Physical examination reveals gynecomastia (bilateral breast tissue enlargement), reduced testicular volume (8 mL), and sparse body hair. Laboratory investigations show: total testosterone 180 ng/dL (normal: 300–1000 ng/dL), free testosterone 2.1 pg/mL (normal: 8.3–24 pg/mL), LH 2.4 mIU/mL (normal: 1.7–8.6 mIU/mL), FSH 3.2 mIU/mL (normal: 1.5–12.4 mIU/mL), and prolactin 68 ng/mL (normal: 2–18 ng/mL). MRI pituitary shows a 12 mm sellar mass. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Prolactinoma with Secondary Hypogonadism ### Clinical Presentation **Key Point:** The constellation of **elevated prolactin + low testosterone + low-normal gonadotropins + pituitary mass on imaging** is diagnostic of prolactinoma causing secondary hypogonadism. ### Hormonal Pattern Analysis | Parameter | Patient Value | Normal Range | Interpretation | |-----------|---------------|--------------|----------------| | Total testosterone | 180 ng/dL | 300–1000 | Low | | Free testosterone | 2.1 pg/mL | 8.3–24 | Low | | LH | 2.4 mIU/mL | 1.7–8.6 | Low-normal (inappropriately low for testosterone) | | FSH | 3.2 mIU/mL | 1.5–12.4 | Low-normal | | Prolactin | 68 ng/mL | 2–18 | **Markedly elevated** | | Pituitary MRI | 12 mm sellar mass | — | Consistent with adenoma | **High-Yield:** Prolactin >50 ng/mL with a sellar mass on imaging is virtually diagnostic of prolactinoma. Prolactin levels >200 ng/mL are highly specific for prolactinoma (vs. secondary hyperprolactinemia from other causes). ### Mechanism of Hypogonadism in Prolactinoma ```mermaid flowchart TD A[Prolactinoma]:::outcome --> B[Elevated Prolactin]:::outcome B --> C[Inhibition of GnRH secretion]:::action C --> D[Decreased LH pulsatility]:::action D --> E[Low testosterone]:::outcome B --> F[Direct inhibition of testicular steroidogenesis]:::action F --> E E --> G[Erectile dysfunction, gynecomastia, reduced libido]:::outcome ``` **Clinical Pearl:** Prolactin has dual suppressive effects on testosterone: 1. **Central:** Inhibits GnRH release → ↓ LH → ↓ testosterone 2. **Peripheral:** Direct inhibition of Leydig cell steroidogenesis Both mechanisms result in low testosterone despite low-normal (not elevated) LH — a hallmark of prolactinoma-induced hypogonadism. ### Why This Is Secondary Hypogonadism **Mnemonic:** **SHAT** = **S**econdary hypogonadism has **H**ypothalamic or **A**nterior pituitary **T**rouble. - **Low testosterone** + **low-normal LH/FSH** = central (secondary) hypogonadism - If primary testicular failure: testosterone would be low but LH/FSH would be **high** (compensatory) - This patient's gonadotropins are inappropriately low for his testosterone level → pituitary/hypothalamic cause ### Clinical Features Explained | Finding | Mechanism | |---------|----------| | Erectile dysfunction | Low free testosterone | | Decreased libido | Low testosterone | | Gynecomastia | Elevated prolactin + relative estrogen excess (↓ testosterone, ↑ aromatization) | | Reduced testicular volume | Prolonged low LH/FSH → Leydig and Sertoli cell atrophy | | Sparse body hair | Low testosterone | ### Diagnostic Confirmation 1. **Prolactin >50 ng/mL** + **sellar mass on MRI** = prolactinoma until proven otherwise 2. **Low testosterone + low-normal LH** = secondary hypogonadism 3. **Dynamic testing (if needed):** TRH stimulation test shows paradoxical prolactin rise in prolactinoma ### Management 1. **First-line:** Dopamine agonists (bromocriptine, cabergoline) 2. **Goal:** Suppress prolactin, restore GnRH pulsatility, normalize testosterone 3. **Surgery:** Reserved for dopamine-resistant tumors or mass effect symptoms (visual field defect, headache)