Unit Converter
Thyroglobulin (Tg)
(Thyroid-Specific Protein - Primary Tumor Marker for Differentiated Thyroid Cancer)
Synonyms
- Thyroglobulin
- Tg
- Serum Tg
- Thyroid tumor marker
- Thyroid-specific glycoprotein
- Thyroid follicular colloid protein
Units of Measurement
- ng/mL
- ng/dL
- ng/100 mL
- ng%
- ng/L
- µg/L
Unit Conversions
1 ng/mL=1 µg/L1\ \text{ng/mL} = 1\ \text{µg/L}1 ng/mL=1 µg/L 1 ng/mL=1000 ng/L1\ \text{ng/mL} = 1000\ \text{ng/L}1 ng/mL=1000 ng/L 1 ng/mL=100 ng/dL1\ \text{ng/mL} = 100\ \text{ng/dL}1 ng/mL=100 ng/dL \text{ng%} = \text{ng/dL}
Description
Thyroglobulin (Tg) is a large glycoprotein (MW ~660 kDa) produced exclusively by thyroid follicular cells, both normal and malignant.
Key Point
Because Tg is thyroid-specific, its presence after total thyroidectomy + radioiodine ablation (RAI) indicates:
- Residual thyroid tissue
- Recurrent disease
- Metastatic differentiated thyroid cancer (DTC)
Tg is the primary tumor marker for:
- Papillary thyroid carcinoma (PTC)
- Follicular thyroid carcinoma (FTC)
Physiological Role
- Precursor for thyroid hormone synthesis (T4, T3)
- Stores iodinated tyrosine residues in colloid
- Released during thyroid inflammation, manipulation, or tumor activity
Clinical Significance
HIGH THYROGLOBULIN
1. Differentiated Thyroid Cancer (DTC) - Most Important
Elevated Tg after thyroidectomy suggests:
- Persistent disease
- Lymph node recurrence
- Distant metastases (lungs, bone)
ATA / NCCN Interpretation
| Tg Value (suppressed) | Likely Interpretation |
| <0.2 ng/mL | Excellent response (post-thyroidectomy + RAI) |
| 0.2 – 1 ng/mL | Indeterminate |
| >1 ng/mL | Biochemical incomplete response |
| >10 ng/mL | Strong suspicion for recurrence |
| >30 ng/mL | High risk – evaluate for structural disease |
Stimulated Tg
| Stimulated Tg | Meaning |
| <1 ng/mL | Excellent response |
| >2 ng/mL | Suggestive of disease |
| >10 ng/mL | High probability of recurrence/metastases |
2. Benign Thyroid Conditions
- Multinodular goiter
- Graves’ disease
- Thyroiditis (Hashimoto, subacute)
- Post-FNA / manipulation of the gland
Levels often increase temporarily.
3. Iodine Deficiency
Tg rises as thyroid mass increases.
4. Thyroid Injury
- Surgery
- RAI therapy (temporary spike)
- Trauma
LOW / UNDETECTABLE Tg
Normal individuals with intact thyroid:
- Tg typically 10–50 ng/mL (lab dependent)
After total thyroidectomy:
- Undetectable Tg → excellent therapeutic response
- Indicates absence of functioning thyroid tissue
Reference Intervals
(Tietz 8E + ATA + Mayo + ARUP)
Normal
- Up to 50 ng/mL
(Can vary from 1–55 ng/mL based on thyroid size and TSH levels)
Post-Thyroidectomy Targets
- Suppressed Tg <0.2 ng/mL → Excellent response (ATA)
- Stimulated Tg <1 ng/mL
Critical Interpretation
- Any detectable Tg after thyroidectomy/ablation requires evaluation
- Rising Tg trend = more important than single value
Anti-Thyroglobulin Antibodies (TgAb) - VERY IMPORTANT
TgAb interfere with Tg assays and cause falsely low or undetectable Tg.
Rules
- ALWAYS measure TgAb with Tg
- If TgAb positive → rely on TgAb trend, NOT Tg value
- Rising TgAb = recurrence
- Falling TgAb = remission
Diagnostic Uses
1. Monitoring Differentiated Thyroid Cancer
- Postoperative surveillance
- Detecting early recurrence
- Monitoring metastatic disease
- Guiding RAI therapy decisions
2. Evaluating Thyroid Tissue Remnant
Tg rises if tissue remains after surgery.
3. Thyroiditis
Elevated Tg during inflammatory destruction.
4. Iodine Deficiency Assessment
Population-based studies use Tg as marker of goiter and thyroid stress.
Analytical Notes
- Immunometric assays (IMAs) commonly used
- Mass spectrometry (LC–MS/MS) avoids antibody interference
- TgAb must always be measured
- TSH suppression status affects Tg levels
- Avoid testing soon after:
- FNA (up to 3 weeks)
- Surgery
- RAI therapy (transient spikes)
- FNA (up to 3 weeks)
Clinical Pearls
- Tg trend is more important than absolute value.
- Undetectable Tg with rising TgAb = suspicious for recurrence.
- Tg >10 ng/mL after thyroidectomy almost always indicates disease.
- Stimulated Tg has higher sensitivity than suppressed Tg.
- LC–MS/MS Tg is preferred when TgAb are positive.
Interesting Fact
Thyroglobulin is among the largest human proteins (≈660 kDa) and can store enough iodinated precursors to supply thyroid hormones for several weeks.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Thyroid Markers
- American Thyroid Association (ATA) Guidelines for Differentiated Thyroid Cancer (2015/2022)
- NCCN Thyroid Cancer Guidelines
- Mayo Clinic Laboratories - Thyroglobulin (Tg)
- ARUP Consult - Thyroid Tumor Markers
- Endocrine Society Thyroid Testing Principles