A 48-year-old woman with newly diagnosed hypercholesterolaemia (total cholesterol 320 mg/dL, LDL-C 240 mg/dL) is started on atorvastatin 40 mg daily. After 6 weeks, repeat lipid panel shows minimal change (LDL-C 220 mg/dL). Her compliance is confirmed. You suspect statin resistance. Which investigation would be most appropriate to identify a secondary cause of statin resistance?

Explanation

## Evaluation of Statin Resistance ### Definition and Clinical Significance **Key Point:** Statin resistance is defined as failure to achieve ≥50% reduction in LDL-C despite high-dose statin therapy with confirmed adherence. This patient shows only ~8% reduction in LDL-C after 6 weeks on atorvastatin 40 mg, indicating true statin resistance. ### Causes of Statin Resistance Statin resistance can be classified into: | Category | Cause | Investigation | |----------|-------|---------------| | **Primary (genetic)** | LDLR mutations, APOB mutations, PCSK9 mutations | Gene sequencing | | **Secondary (acquired)** | Hypothyroidism, nephrotic syndrome, liver disease, drug interactions | TSH, free T4, urinalysis, LFTs, drug review | | **Pharmacokinetic** | Poor absorption, drug metabolism polymorphisms | Clinical assessment | | **Non-adherence** | Patient non-compliance | Pill count, pharmacy refill history | ### Why TSH and Free T4 are Correct **High-Yield:** Hypothyroidism is a **common, reversible secondary cause of statin resistance** that must be ruled out before attributing resistance to primary genetic defects. Here's why this is the most appropriate next investigation: 1. **Prevalence** — Hypothyroidism affects ~5–10% of the general population and significantly impairs statin efficacy 2. **Mechanism** — Thyroid hormone regulates: - LDL receptor expression (↓ in hypothyroidism) - HMG-CoA reductase activity (↑ in hypothyroidism) - Cholesterol clearance from blood 3. **Reversibility** — Correcting hypothyroidism with levothyroxine restores statin responsiveness 4. **Clinical pearls** — Hypothyroid patients often have: - Elevated cholesterol (due to ↓ LDL receptor expression) - Poor statin response until euthyroid - Other features: fatigue, weight gain, cold intolerance, bradycardia 5. **Cost-effective** — TSH/free T4 is inexpensive and readily available **Clinical Pearl:** Always screen for hypothyroidism in statin-resistant patients before pursuing expensive genetic testing or escalating therapy. ### Diagnostic Algorithm for Statin Resistance ```mermaid flowchart TD A[Statin resistance suspected]:::outcome --> B[Confirm adherence + adequate dosing]:::action B --> C{Adherence confirmed?}:::decision C -->|No| D[Counsel on compliance]:::action C -->|Yes| E[Screen for secondary causes]:::action E --> F[TSH + free T4]:::action E --> G[Urinalysis for proteinuria]:::action E --> H[LFTs]:::action F --> I{Hypothyroid?}:::decision I -->|Yes| J[Start levothyroxine]:::action I -->|No| K[Consider primary genetic FH]:::action K --> L[LDLR gene sequencing]:::action ``` **Mnemonic:** **SHRED** = **S**econdary causes first → **H**ypothyroidism → **R**enal disease → **E**ther drug interactions → **D**efective genes (last resort)