How do you interpret Thyroid-Stimulating Hormone (TSH) levels?

Interpreting TSH Levels: A Clinical Guide

The interpretation of TSH levels should be based on established reference ranges, with normal values typically between 0.45-4.5 mIU/L, though slight adjustments may be needed for older adults where the upper limit may extend to 5.9 mIU/L. 1

Normal TSH Values and Reference Ranges

• The 95% confidence interval for TSH reference values in adults is 0.45-4.12 mIU/L 1
• In older adults (70-79 years), the reference range becomes slightly broader at 0.4-5.9 mIU/L 1
• A normal TSH indicates euthyroidism with an accuracy of >99%, making it an excellent screening test for thyroid function 1, 2
• Laboratory reference intervals are based on statistical distribution of TSH levels across the general population rather than association with symptoms or outcomes 3

Interpreting Abnormal TSH Values

Elevated TSH

• TSH >10 mIU/L with normal free T4 indicates subclinical hypothyroidism with high risk of progression to overt hypothyroidism 4
• TSH between 4.5-10 mIU/L with normal free T4 represents mild subclinical hypothyroidism 3, 4
• Elevated TSH with low free T4 indicates overt hypothyroidism 3
• About 37% of persons with subclinical hypothyroidism spontaneously revert to euthyroid state without intervention 3

Low TSH

• TSH <0.1 mIU/L with normal free T4 and T3 indicates subclinical hyperthyroidism 3
• TSH between 0.1-0.45 mIU/L represents "low but detectable" subclinical hyperthyroidism 3
• TSH <0.1 mIU/L with elevated T4 or T3 indicates overt hyperthyroidism 3
• When TSH is undetectable (<0.04 mIU/L), thyrotoxicosis is present in 97% of cases (excluding patients on thyroid hormone therapy) 5
• Approximately 25% of persons with subclinical hyperthyroidism revert to euthyroid state without medical intervention 3

Clinical Approach to Abnormal TSH Results

Confirming Abnormal Results

• Repeat thyroid function tests after 3-6 months to confirm persistent abnormalities before making treatment decisions 3, 4
• Include free T4 measurement when TSH is abnormal to distinguish between subclinical and overt thyroid dysfunction 1
• Consider factors that may affect TSH levels, including acute illness, certain medications, and pregnancy 3

Management Based on TSH Levels

• For TSH >10 mIU/L: Levothyroxine therapy is recommended even without symptoms 4
• For TSH 4.5-10 mIU/L: Routine levothyroxine treatment is not recommended unless symptoms compatible with hypothyroidism are present 4
• For TSH <0.1 mIU/L: Monitor for progression to overt hyperthyroidism and increased risk of atrial fibrillation 3
• For pregnant women or those planning pregnancy: Treat with levothyroxine to restore TSH to reference range regardless of TSH level 4

Special Considerations and Pitfalls

• TSH alone has high sensitivity but low positive predictive value (12%) for hyperthyroidism in older adults; adding T4 measurement increases predictive value to 67% 6
• False-positive results can occur due to TSH secretion variability, acute illness, or medication effects 3
• Heterophilic antibodies may cause spuriously elevated TSH values in some immunometric assays 1
• Central hypothyroidism (pituitary or hypothalamic disease) may present with normal or low TSH despite hypothyroidism 1, 7
• Non-thyroidal illness (sick euthyroid syndrome) can affect thyroid function tests without actual thyroid dysfunction 7

Follow-up and Monitoring

• In adults with primary hypothyroidism, monitor TSH 6-8 weeks after any dosage change 8
• For stable patients on appropriate replacement therapy, evaluate clinical and biochemical response every 6-12 months 8
• Target TSH values for treated hypothyroidism are between 0.25-2.0 mIU/L, avoiding values ≤0.10 mIU/L 1
• In differentiated thyroid cancer following complete thyroid ablation, the target TSH value is ≤0.10 mIU/L 1

Remember that symptoms of thyroid dysfunction are often non-specific and extremely prevalent in the general population, making biochemical confirmation essential for diagnosis 2.