A 52-year-old man on levothyroxine replacement for hypothyroidism presents for routine follow-up. His symptoms are well-controlled, and he is clinically euthyroid. Which investigation is most appropriate to assess adequacy of thyroid replacement and guide dose adjustment?

Explanation

## Monitoring Adequacy of Thyroid Replacement **Key Point:** Serum free T4 and TSH are the investigations of choice for monitoring adequacy of levothyroxine replacement. The goal is to maintain TSH in the target range (typically 0.5–2.5 mIU/L) with free T4 in the upper-normal range. ### Rationale for Free T4 + TSH Monitoring **High-Yield:** In patients on levothyroxine: - **TSH** reflects the adequacy of replacement and is the most sensitive indicator of over- or under-replacement - **Free T4** confirms that the free hormone level is appropriate and not excessively high (which increases cardiovascular and bone loss risk) - This combination prevents both under-treatment (persistent hypothyroid symptoms) and over-treatment (iatrogenic thyrotoxicosis) **Clinical Pearl:** The TSH set-point changes with age and comorbidities. In elderly patients or those with cardiac disease, a slightly higher TSH target (1.0–3.0 mIU/L) may be preferred to avoid over-replacement-related atrial fibrillation and osteoporosis. ### Monitoring Protocol for Levothyroxine Replacement ```mermaid flowchart TD A[Patient on levothyroxine]:::outcome --> B[Measure TSH + free T4]:::action B --> C{TSH in target range?}:::decision C -->|Yes, FT4 normal| D[Continue current dose]:::action C -->|TSH high, FT4 low| E[Increase levothyroxine dose]:::action C -->|TSH low, FT4 high| F[Decrease levothyroxine dose]:::urgent E --> G[Recheck TSH + FT4 in 6-8 weeks]:::action F --> G D --> H[Routine follow-up annually]:::action ``` ### Why Other Investigations Are Inappropriate | Investigation | Why Not Used for Monitoring | |---|---| | **Total T4 and total T3** | Affected by binding proteins (estrogen, pregnancy); do not reflect bioavailable hormone. Free T4 is more accurate. | | **TPO and thyroglobulin antibodies** | Measure autoimmune status, not thyroid function. Antibody titers do not change with levothyroxine dose and do not guide therapy. | | **TSI (thyroid-stimulating immunoglobulin)** | Specific for Graves' disease (hyperthyroidism), not relevant in hypothyroidism management. | **Mnemonic:** **FT4-TSH Pair** — Free T4 for hormone adequacy; TSH for feedback control. Together they guide levothyroxine dosing. **Warning:** Do not use total T4/T3 for monitoring — they are unreliable in patients on replacement therapy due to variable protein binding. Always use free T4.