What are the normal reference ranges for thyroid labs in Canada, including Thyroid-Stimulating Hormone (TSH), free T4, and free T3?

Normal Reference Ranges for Thyroid Labs in Canada

The recommended normal reference ranges for thyroid function tests are TSH 0.45-4.5 mIU/L, with free T4 and free T3 interpreted using laboratory-specific reference intervals. 1

TSH (Thyroid-Stimulating Hormone)

• The standard TSH reference range is 0.45-4.5 mIU/L, based on data from disease-free populations excluding those with thyroid antibodies or known thyroid disease 1
• Values below 0.1 mIU/L are generally considered low, while values above 6.5 mIU/L are considered elevated in screening contexts 2
• The lower limit (2.5th percentile) remains consistent at approximately 0.5 mIU/L across all age groups 3
• The upper limit increases with age: approximately 3.75 mIU/L at age 40, rising to 5.0 mIU/L by age 90, though this has minimal clinical impact except in patients over 85 years 3

Free T4 (Thyroxine)

• Free T4 must be interpreted using your specific laboratory's established reference range, as values vary significantly between different immunoassay platforms and manufacturers 4, 5
• Each laboratory should independently establish reference intervals using their specific analyzer and reagent system 5
• Free T4 should always be interpreted in conjunction with TSH levels for accurate assessment 4

Free T3 (Triiodothyronine)

• Free T3 reference ranges are laboratory-specific and should be obtained from your testing facility 1
• Normal free T3 with abnormal TSH indicates subclinical thyroid dysfunction 4

Diagnostic Interpretation Algorithm

• If TSH >4.5 mIU/L with low free T4: overt primary hypothyroidism 1
• If TSH >4.5 mIU/L with normal free T4: subclinical hypothyroidism 2, 1
• If TSH <0.45 mIU/L with elevated free T4 and/or free T3: overt hyperthyroidism 1
• If TSH <0.45 mIU/L with normal free T4 and free T3: subclinical hyperthyroidism 1

Critical Pitfalls to Avoid

• Non-thyroidal illness can cause low TSH, low T3, and normal/low T4 without actual thyroid dysfunction—do not diagnose thyroid disease in acutely ill hospitalized patients 1, 4
• Medications that suppress TSH without causing true hyperthyroidism include dopamine, glucocorticoids, and dobutamine 1, 4
• Amiodarone can significantly alter thyroid hormone levels in both directions 4, 5
• Iodine exposure from CT contrast can transiently impact thyroid function tests—wait 4-6 weeks after contrast studies before interpreting results 1, 4
• Pregnancy causes physiologically low TSH with normal free hormone levels, particularly in the first trimester 1
• Heterophilic antibodies against mouse proteins can cause falsely elevated TSH in some assays 2
• Recent levothyroxine dose adjustments require 6-8 weeks to reach steady state before accurate interpretation 2
• Recovery from severe illness or destructive thyroiditis can cause transient TSH elevation 2
• Between-assay differences at TSH concentrations around 4.0 mIU/L can vary by approximately 1 mIU/L, potentially affecting clinical decisions 3

Quality Control Considerations

• The TSH assay used should have a functional sensitivity of at least 0.02 mIU/L, independently established by each laboratory 2
• Laboratories must engage in appropriate quality control procedures to ensure accurate and reproducible results 2