A 50-year-old man undergoes total thyroidectomy for papillary thyroid carcinoma (PTC). Histopathology confirms PTC with vascular invasion and extrathyroidal extension. Six weeks post-operatively, serum thyroglobulin (Tg) is elevated at 8 ng/mL (normal

Question

A 50-year-old man undergoes total thyroidectomy for papillary thyroid carcinoma (PTC). Histopathology confirms PTC with vascular invasion and extrathyroidal extension. Six weeks post-operatively, serum thyroglobulin (Tg) is elevated at 8 ng/mL (normal <0.5 ng/mL) despite TSH suppression. Which is the most appropriate next investigation to detect metastatic disease?

Accepted Answer

C. Radioiodine whole-body scan (I-131 WBS) after thyroid hormone withdrawal or recombinant human TSH (rhTSH) stimulation. ## Investigation of Choice for Metastatic PTC Detection **Key Point:** Radioiodine whole-body scan (I-131 WBS) is the gold standard investigation for detecting metastatic papillary thyroid carcinoma when serum thyroglobulin is elevated post-thyroidectomy. ### Thyroglobulin as a Tumor Marker in PTC 1. **Highly sensitive and specific** — Tg is produced only by thyroid follicular cells; elevated Tg post-thyroidectomy indicates residual or metastatic thyroid tissue. 2. **Prognostic significance** — Tg >1 ng/mL on TSH suppression or >2 ng/mL on TSH stimulation suggests metastatic disease in >90% of cases. 3. **TSH stimulation enhances sensitivity** — Tg is TSH-dependent; stimulation via hormone withdrawal or rhTSH increases Tg secretion, improving detection of small metastases. ### Why I-131 WBS is the Investigation of Choice | Feature | I-131 WBS | Ultrasound | Chest X-ray | Anti-Tg | |---|---|---|---|---| | **Detects iodine-avid metastases** | Yes (gold standard) | No | No | No | | **Whole-body coverage** | Yes | Neck only | Chest only | N/A | | **Sensitivity for metastases** | 80–90% | 50–70% | 30–40% | N/A | | **Therapeutic potential** | Yes (I-131 ablation) | No | No | No | | **Functional information** | Yes (iodine uptake) | Anatomic only | Anatomic only | Antibody interference | **Clinical Pearl:** I-131 WBS should be performed after TSH stimulation (either by thyroid hormone withdrawal for 4–6 weeks or rhTSH injection) to maximize sensitivity. rhTSH is preferred in clinical practice because it avoids hypothyroid symptoms and is faster. **High-Yield:** Elevated Tg with negative I-131 WBS suggests non-iodine-avid metastases; in such cases, FDG-PET/CT becomes the next investigation of choice. **Mnemonic:** **STIR** — Stimulate TSH, I-131 scan, Residual/metastatic disease detection, Radioiodine therapy. ### Why Other Investigations Are Not First-Line 1. **Serum thyroglobulin antibodies (anti-Tg)** — Measured to detect antibody interference with Tg assay (false-negative Tg in ~5–10% of patients). Does not detect metastases; only explains discordant Tg results. 2. **Ultrasound of the neck** — Useful for detecting neck lymph node metastases but provides limited whole-body assessment. Should be performed but is complementary to I-131 WBS, not a substitute. 3. **Chest X-ray and abdominal ultrasound** — Low sensitivity for small metastases. Useful for detecting large pulmonary or hepatic lesions but miss micrometastases detected by I-131 WBS. ### Algorithm for Elevated Tg Post-Thyroidectomy ```mermaid flowchart TD A[PTC post-thyroidectomy]:::outcome --> B[Serum Tg elevated]:::outcome B --> C[Check anti-Tg antibodies]:::action C --> D{Anti-Tg positive?}:::decision D -->|Yes| E[Antibody interference likely
Repeat Tg assay or use
alternative method]:::action D -->|No| F[Proceed to I-131 WBS
after TSH stimulation]:::action F --> G{I-131 WBS positive?}:::decision G -->|Yes| H[Iodine-avid metastases
I-131 therapy]:::action G -->|No| I[Non-iodine-avid metastases
FDG-PET/CT]:::action ``` [cite:Robbins 10e Ch 24] ![Thyroid Neoplasms diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/4115.webp)

Answer

A. Chest X-ray and abdominal ultrasound

Answer

B. Serum thyroglobulin antibodies (anti-Tg) measurement

Answer

D. Ultrasound of the neck

Explanation

Investigation of Choice for Metastatic PTC Detection

Thyroglobulin as a Tumor Marker in PTC

Why I-131 WBS is the Investigation of Choice

Why Other Investigations Are Not First-Line

Algorithm for Elevated Tg Post-Thyroidectomy

Practice similar questions

Join our NEET PG community

Feature	I-131 WBS	Ultrasound	Chest X-ray	Anti-Tg
Detects iodine-avid metastases	Yes (gold standard)	No	No	No
Whole-body coverage	Yes	Neck only	Chest only	N/A
Sensitivity for metastases	80–90%	50–70%	30–40%	N/A
Therapeutic potential	Yes (I-131 ablation)	No	No	No
Functional information	Yes (iodine uptake)	Anatomic only	Anatomic only	Antibody interference