Diagnostic Confirmation of Mild to Moderate Hyperthyroidism
Confirm hyperthyroidism by measuring serum TSH first, then if suppressed (<0.1 mIU/L for moderate or 0.1-0.45 mIU/L for mild), measure free T4 and total T3 or free T3 to distinguish between subclinical (mild) and overt (moderate) disease. 1
Initial Diagnostic Algorithm
Step 1: Measure Serum TSH
- TSH is the first-line screening test with >98% sensitivity for detecting thyroid dysfunction 1
- A suppressed TSH (<0.45 mIU/L) indicates possible hyperthyroidism 2
- Mild hyperthyroidism (subclinical): TSH 0.1-0.45 mIU/L 2
- Moderate hyperthyroidism: TSH <0.1 mIU/L 2, 1
Step 2: Confirm with Repeat Testing
- Always repeat abnormal TSH measurements before making treatment decisions 1
- For TSH 0.1-0.45 mIU/L without cardiac symptoms: repeat within 3 months 2
- For TSH 0.1-0.45 mIU/L with atrial fibrillation, cardiac disease, or serious medical conditions: repeat within 2 weeks 2
- For TSH <0.1 mIU/L: repeat measurement along with FT4 and T3 within 4 weeks 2
- If cardiac symptoms or arrhythmias present with TSH <0.1 mIU/L: perform testing urgently 2, 1
Step 3: Measure Free T4 and T3
- If TSH remains suppressed on repeat testing, measure free T4 and either total T3 or free T3 2, 1
- Mild/subclinical hyperthyroidism: Low TSH (0.1-0.45 mIU/L or <0.1 mIU/L) with normal FT4 and T3 2
- Moderate/overt hyperthyroidism: Low TSH (<0.1 mIU/L) with elevated FT4 and/or T3 2, 3
Severity Classification
Mild Hyperthyroidism (Subclinical)
- TSH between 0.1-0.45 mIU/L with normal FT4 and T3 2
- May be asymptomatic or have minimal symptoms 2
- Requires monitoring at 3-12 month intervals if stable 2
Moderate Hyperthyroidism (Overt)
- TSH <0.1 mIU/L with elevated FT4 and/or T3 3
- Often symptomatic with tachycardia, tremors, weight loss, heat intolerance 3
- Requires prompt treatment consideration 2
Determining Etiology After Confirmation
Once biochemical hyperthyroidism is confirmed, establish the underlying cause:
- Measure TSH-receptor antibodies to identify Graves' disease (most common cause, 70% of cases) 4
- Perform thyroid ultrasonography to evaluate for nodules 4
- Consider radioactive iodine uptake and scan if nodules present or etiology unclear 2
- The scan distinguishes between Graves' disease (diffuse uptake), toxic nodular goiter (focal uptake), and destructive thyroiditis (low uptake) 2
Evaluating for Complications
Cardiovascular Assessment
- Screen for atrial fibrillation, which is more common in hyperthyroid patients 2, 1
- Assess for pulmonary artery hypertension, which can occur and lead to right ventricular dilatation 2, 1
- In severe, long-standing cases, evaluate for heart failure despite typically increased cardiac contractility 2, 1
- Cardiovascular complications are the chief cause of death after treatment in patients >50 years 2
Skeletal Assessment
- Consider bone mineral density testing in postmenopausal women with prolonged subclinical hyperthyroidism (TSH <0.1 mIU/L), as they are at increased risk for bone loss and fractures 2, 1
Common Pitfalls to Avoid
- False positives: Severe non-thyroid illness can cause falsely suppressed TSH 1
- Iodine exposure: Patients with nodular thyroid disease may develop overt hyperthyroidism when exposed to excess iodine (e.g., radiographic contrast agents) and require special monitoring 2, 1
- Medication effects: Amiodarone, tyrosine kinase inhibitors, and immune checkpoint inhibitors can cause hyperthyroidism 4
- Age-specific considerations: Elderly patients may have atypical presentations and are at higher risk for cardiovascular complications 2
- Pregnancy: Untreated or inadequately treated hyperthyroidism increases risk of maternal heart failure, spontaneous abortion, preterm birth, and fetal hyperthyroidism 5, 6
Monitoring Strategy
For Mild Hyperthyroidism (TSH 0.1-0.45 mIU/L)
- If stable without cardiac disease: monitor TSH, FT4, and T3 every 3-12 months 2
- Continue monitoring until TSH normalizes or condition is confirmed stable 2
For Moderate Hyperthyroidism (TSH <0.1 mIU/L)