A 38-year-old woman with a 3-year history of treated hypothyroidism (on levothyroxine 75 mcg daily) presents with persistent fatigue, myalgia, and weight gain despite reported adherence to medication. Her TSH is 8.2 mIU/L (target <2.5), and free T4 is 0.9 ng/dL (normal 0.8–1.8). She denies any new medications, and her calcium and vitamin B12 are normal. On direct questioning, she admits to taking her levothyroxine with breakfast and a cup of tea. What is the most likely reason for inadequate response to therapy?

Explanation

## Diagnosis: Levothyroxine Malabsorption Due to Drug–Food Interaction ### Key Point: Timing and Drug Interactions in Levothyroxine Absorption **High-Yield:** Levothyroxine absorption is optimal on an **empty stomach, 30–60 minutes before food or other medications**. Taking it with breakfast and tea significantly reduces bioavailability and therapeutic efficacy. ### Mechanism of Reduced Absorption | Interfering Agent | Mechanism | Effect on Absorption | |-------------------|-----------|---------------------| | Tea (polyphenols, tannins) | Chelation of levothyroxine in GI lumen | ↓ 20–30% | | Breakfast (calcium, iron, fiber) | Competition for intestinal transporters; chelation | ↓ 20–50% | | Milk/dairy | High calcium content | ↓ 30–40% | | Iron supplements | Chelation complex formation | ↓ 50–60% | | Proton pump inhibitors | Reduced gastric acidity → ↓ dissolution | ↓ 15–25% | **Clinical Pearl:** Even though the patient reports "adherence," she is not achieving therapeutic absorption. This is a common cause of apparent treatment failure in hypothyroidism. ### Why This Patient's TSH Remains Elevated ```mermaid flowchart TD A[Levothyroxine 75 mcg prescribed]:::action --> B[Taken with breakfast + tea] B --> C[Tea polyphenols & food chelate drug]:::outcome C --> D[Absorption reduced to ~50% of dose]:::outcome D --> E[Effective dose ≈ 37.5 mcg]:::outcome E --> F[Insufficient T4 to suppress TSH]:::outcome F --> G[TSH remains elevated at 8.2]:::outcome G --> H[Persistent hypothyroid symptoms]:::outcome ``` **Key Point:** The patient's free T4 (0.9 ng/dL) is at the lower end of normal, and TSH is elevated—indicating **inadequate replacement**, not treatment resistance or a new pathology. ### Clinical Approach to Suspected Malabsorption 1. **Verify timing:** Levothyroxine must be taken on an empty stomach. 2. **Eliminate interfering substances:** No food, tea, coffee, milk, or iron for ≥30–60 minutes after dose. 3. **Recheck TSH in 6–8 weeks** after correcting timing. 4. **If TSH still elevated:** Consider malabsorption disorders (celiac disease, atrophic gastritis) or other drug interactions. **High-Yield:** Simply correcting the timing of administration often resolves the problem without increasing the levothyroxine dose. [cite:Harrison 21e Ch 405; KD Tripathi 8e Ch 56]