A 32-year-old woman presents with acute onset severe thyroid pain, fever, and dysphagia. On examination, the thyroid is tender and enlarged. TSH is suppressed (0.1 mIU/L) with elevated free T4 (18 pg/dL). Thyroid peroxidase (TPO) antibodies are negative. Which investigation is most specific for confirming the diagnosis of acute suppurative thyroiditis?

Explanation

## Diagnosis of Acute Suppurative Thyroiditis **Key Point:** Acute suppurative (bacterial) thyroiditis is a rare but serious condition requiring microbiological confirmation and identification of the causative organism for targeted antibiotic therapy. ### Why FNAC with Gram Stain and Culture is the Investigation of Choice **High-Yield:** FNAC with Gram stain and culture is the most specific investigation because it: 1. **Directly identifies the organism** — allows Gram staining and bacterial culture 2. **Guides antibiotic therapy** — enables culture-and-sensitivity testing 3. **Confirms suppuration** — cytology shows neutrophilic infiltration and bacterial organisms 4. **Minimally invasive** — safer than open biopsy in an acutely inflamed gland ### Clinical Presentation Context The clinical triad of acute suppurative thyroiditis includes: - Severe thyroid pain and tenderness - Fever and systemic toxicity - Rapid thyroid enlargement The **suppressed TSH with elevated free T4** reflects thyroiditis-induced thyroid hormone release (thyrotoxicosis), not Graves' disease (negative TPO antibodies rules out autoimmune thyroiditis). ### Differential Investigation Roles | Investigation | Role | Diagnostic Yield | |---|---|---| | **FNAC + Gram/Culture** | Microbiological confirmation, organism identification | **Highest — gold standard** | | Ultrasound with Doppler | Assesses gland architecture, guides FNAC | Supportive only | | Radionuclide scan | Shows suppressed uptake in thyroiditis | Non-specific | | Thyroglobulin levels | Elevated in any thyroiditis | Non-specific | **Clinical Pearl:** Acute suppurative thyroiditis often follows upper respiratory infection or dental procedures. *Staphylococcus aureus* is the most common organism, followed by *Streptococcus pyogenes* and gram-negative bacteria. Culture results directly inform antibiotic selection. **Warning:** Do NOT confuse with acute viral thyroiditis (De Quervain's) — that is self-limited, has negative cultures, and does not require antibiotics.