What TSH (Thyroid-Stimulating Hormone) level indicates hyperthyroidism?

TSH Values Indicating Hyperthyroidism

Hyperthyroidism is characterized by a TSH level of less than 0.1 mIU/L, indicating significant thyroid hyperfunction. 1

Classification of Hyperthyroidism Based on TSH and Thyroid Hormone Levels

Hyperthyroidism can be categorized into different subtypes based on laboratory values:

• Overt Hyperthyroidism: Low/suppressed TSH (< 0.1 mIU/L) + elevated Free T4 and/or Free T3
• Subclinical Hyperthyroidism: Low/suppressed TSH + normal Free T4 and Free T3
• T3-Toxicosis: Low TSH + normal Free T4 + elevated Free T3 1

Subclinical hyperthyroidism can be further classified based on the degree of TSH suppression:

• Mild: TSH between 0.1-0.45 mIU/L
• Severe: TSH < 0.1 mIU/L 2

Diagnostic Approach to Low TSH

When encountering a low TSH level, it's important to follow a systematic approach:

1. Initial finding: TSH < 0.45 mIU/L (below reference range)
2. Next step: Measure Free T4 and Free T3 levels on the same sample
3. Interpretation:
   • TSH < 0.1 mIU/L with elevated Free T4/T3: Overt hyperthyroidism
   • TSH < 0.1 mIU/L with normal Free T4/T3: Severe subclinical hyperthyroidism
   • TSH 0.1-0.45 mIU/L with normal Free T4/T3: Mild subclinical hyperthyroidism 1, 2

Important Clinical Considerations

• A low TSH value alone has high sensitivity and specificity for hyperthyroidism but a low positive predictive value (12%) for the diagnosis 3
• Adding Free T4 measurement significantly improves the positive predictive value to approximately 67% 3
• For patients with TSH levels below 0.05 mIU/L, knowledge of Free T4 is necessary for proper management in all cases 4

Common Pitfalls to Avoid

1. Assuming all low TSH values indicate hyperthyroidism: Other conditions and medications can cause low TSH without hyperthyroidism 5

2. Overdiagnosis in elderly patients: About 3.9% of ambulatory persons over 60 years have low TSH levels (< 0.1 mIU/L), but most are not hyperthyroid 3

3. Misinterpreting TSH in patients on thyroid hormone therapy: Patients on levothyroxine may have suppressed TSH without clinical hyperthyroidism 1

4. Ignoring age-specific reference ranges: The normal TSH reference interval may be slightly broader in elderly patients (0.4-5.9 mIU/L for ages 70-79) 6

5. Missing central hypothyroidism: Rarely, a low TSH may indicate central (pituitary or hypothalamic) hypothyroidism rather than hyperthyroidism 6

Special Populations

• Elderly patients: More cautious interpretation is needed as they have higher prevalence of low TSH without hyperthyroidism and are at increased risk of complications from subclinical hyperthyroidism 1, 3

• Patients with thyroid cancer: TSH suppression therapy intentionally aims for TSH levels between 0.1-0.5 mIU/L or below 0.1 mIU/L depending on disease status 1

• Patients being treated for hyperthyroidism: TSH may remain suppressed despite normalization of Free T4/T3 levels 4

• Patients with pituitary/hypothalamic disease: TSH values may be misleading 4