A 28-year-old man with a known history of Hashimoto's thyroiditis is being evaluated for persistent symptoms despite levothyroxine replacement. His TSH is 2.1 mIU/L (normal: 0.4–4.0) and FT4 is 12 pg/mL (normal: 9–17). He complains of fatigue and poor concentration. Which investigation is most appropriate to assess his thyroid hormone metabolism and peripheral conversion?

Explanation

## Evaluation of Persistent Hypothyroid Symptoms Despite Adequate TSH Suppression **Key Point:** When TSH and FT4 are within normal range but symptoms persist, measurement of T3 and FT3 is essential to assess peripheral conversion and identify T3 deficiency or impaired deiodinase activity. ### Pathophysiology of T3 Deficiency **High-Yield:** Approximately 80% of circulating T3 is derived from peripheral conversion of T4 by deiodinase enzymes (primarily type 1 deiodinase in liver and kidney). Levothyroxine monotherapy provides T4 but depends on adequate conversion to the more biologically active T3. ### Why T3/FT3 Measurement is Indicated 1. **Normal TSH + FT4 but persistent symptoms** → suggests inadequate T3 availability 2. **Impaired conversion** → may occur in: - Chronic illness, malnutrition, stress - Selenium deficiency (cofactor for deiodinase) - Genetic polymorphisms in deiodinase genes - Medications (beta-blockers, glucocorticoids, amiodarone) 3. **T3 measurement** → directly assesses bioavailable thyroid hormone ### Investigation Comparison in Symptomatic Hypothyroidism | Investigation | Purpose | When to Order | |---|---|---| | **TSH + FT4** | Assess adequacy of replacement | Initial and routine monitoring | | **Total T3 + FT3** | Assess peripheral T3 status; identify conversion defect | Persistent symptoms with normal TSH/FT4 | | **Reverse T3 (rT3)** | Assess deiodinase shunting to inactive metabolite | Research/specialized settings; not routine | | **TSI antibodies** | Assess for concurrent Graves' disease | Only if hyperthyroid features emerge | | **Repeat TSH in 6 weeks** | Monitor dose adjustment | After dose change, not for diagnostic assessment | **Clinical Pearl:** In this patient, low-normal or low FT3 despite normal FT4 would explain persistent fatigue and cognitive symptoms. Some patients benefit from combination T4/T3 therapy (levothyroxine + liothyronine) or desiccated thyroid extract, though evidence is mixed. Measuring T3/FT3 guides this decision. **Mnemonic: T4→T3 Conversion** — When TSH is normal but symptoms persist, check T3 and FT3 to assess the final step of thyroid hormone metabolism. [cite:KD Tripathi 8e Ch 31]