Diagnostic Labs for Hyperthyroidism
The primary diagnostic test for hyperthyroidism is serum thyroid stimulating hormone (TSH), followed by free thyroxine (FT4) and free triiodothyronine (FT3) measurements when TSH is low. 1, 2
Initial Laboratory Evaluation
TSH: First-line screening test
Free T4 (FT4): Second-line test when TSH is abnormal
- Elevated in overt hyperthyroidism
- Normal in subclinical hyperthyroidism 1
Free T3 (FT3): Important complementary test
- More sensitive for detecting hyperthyroidism than FT4 alone
- Critical for identifying "T3-toxicosis" (hyperthyroidism with normal T4 but elevated T3) 5
Diagnostic Patterns in Hyperthyroidism
| Condition | TSH | Free T4 | Free T3 | Diagnosis |
|---|---|---|---|---|
| Overt Hyperthyroidism | Low | Elevated | Elevated | Severe hyperthyroidism |
| Subclinical Hyperthyroidism | Low | Normal | Normal | Mild hyperthyroidism |
| T3-Toxicosis | Low | Normal | Elevated | T3-predominant hyperthyroidism |
Additional Tests for Determining Etiology
Once hyperthyroidism is biochemically confirmed, additional tests help determine the underlying cause:
- TSH-receptor antibodies: Positive in Graves' disease 2
- Thyroid peroxidase antibodies: Often positive in autoimmune thyroid disorders 2
- Thyroid ultrasonography: Helps identify nodules, gland enlargement, or increased vascularity 2
- Thyroid scintigraphy (radioiodine uptake): Differentiates between causes of hyperthyroidism
- Increased uptake: Graves' disease, toxic nodular goiter
- Decreased uptake: Thyroiditis, exogenous thyroid hormone 2
Interpretation Considerations
- A TSH value below 0.1 mIU/L has a sensitivity of 98% and specificity of 98% for detecting hyperthyroidism 4
- Using a higher TSH cutoff of 0.3 mIU/L increases sensitivity to 100% but reduces specificity to 92% 4
- Combining low TSH with elevated FT4 provides the highest diagnostic accuracy (sensitivity 94%, specificity 99%) 4
Common Pitfalls to Avoid
- Relying on total T4/T3 measurements: These are affected by changes in thyroid hormone binding proteins and may not accurately reflect thyroid status 6
- Missing T3-toxicosis: Some patients have normal T4 but elevated T3 levels 5
- Overlooking non-thyroidal illness: Severe non-thyroidal diseases can cause false positive TSH results (positive predictive value of low TSH was only 0.24 for hyperthyroidism in hospitalized patients) 3
- Heterophilic antibody interference: Can cause falsely elevated TSH values 7
Special Populations
- Elderly patients: May have atypical presentations and slightly broader TSH reference ranges (0.4-5.9 mIU/L for ages 70-79) 7
- Pregnant women: Require trimester-specific reference ranges
- Patients with Down syndrome: May have overlapping symptoms with hypothyroidism 3
Remember that hyperthyroidism is associated with significant morbidity and mortality, including atrial fibrillation, heart failure, osteoporosis, and neuropsychiatric problems if left untreated 3, 2. Prompt and accurate laboratory diagnosis is essential for initiating appropriate treatment.