What are the recommended thyroid‑stimulating hormone (TSH) and free thyroxine (Free T4) target ranges for treating hypothyroidism in Canada, including adjustments for pregnancy and patients aged 65 years or older?

Canadian Hypothyroidism Medication Targets

For non-pregnant adults with hypothyroidism in Canada, target TSH should be within the normal laboratory reference range (typically 0.4-4.12 mIU/L), with pregnancy requiring trimester-specific targets: <2.5 mIU/L in first trimester, <3.0 mIU/L in second trimester, and <3.5 mIU/L in third trimester. For patients aged 65 years or older, age-adjusted upper TSH limits should be considered, as normal TSH increases with age.

General Adult Population (Non-Pregnant)

The treatment target for hypothyroidism is straightforward: aim for TSH within the normal reference range of your laboratory's third-generation TSH assay 1. If laboratory-specific ranges are unavailable, use 0.45-4.12 mIU/L based on the NHANES III reference population 1.

Key principle: The evidence does not support targeting specific TSH values within the normal range for non-pregnant patients 1. This means aiming for the lower half of normal (e.g., 0.5-2.5 mIU/L) offers no proven clinical benefit over any value within the normal range 1.

Important Caveats:

• Free T4 should be within normal range when TSH is normalized 1
• Avoid targeting low-normal or subnormal TSH levels, as this increases risks of atrial fibrillation and osteoporosis without demonstrated benefit 1
• Monitor TSH at 4-8 weeks after dose changes, then every 6-12 months once stable 1

Pregnancy Targets

Pregnant women require significantly tighter TSH control with trimester-specific targets 1:

Trimester-Specific TSH Targets:

• First trimester: <2.5 mIU/L (Grade B recommendation) 1
• Second trimester: <3.0 mIU/L (Grade C recommendation) 1
• Third trimester: <3.5 mIU/L (Grade C recommendation) 1

Monitoring Schedule:

• Check TSH promptly after conception 1
• Monitor every 4 weeks during first 20 weeks of pregnancy 1
• Check at least once between 26-32 weeks gestation 1
• Expect levothyroxine requirements to increase by approximately 30-50% during pregnancy 2

Clinical pearl: Women already on levothyroxine should increase their dose by 25-30% as soon as pregnancy is confirmed, then titrate based on TSH monitoring 2.

Patients Aged 65 Years and Older

The normal TSH reference range increases with age, and this should be factored into treatment targets 1, 3. While the 2012 AACE/ATA guidelines recommend an upper limit of 4.12 mIU/L if age-based ranges are unavailable 1, emerging evidence suggests higher TSH values may be acceptable in elderly patients.

Age-Adjusted Approach:

• For patients 65-80 years: Consider upper TSH limit of approximately 5.0-6.0 mIU/L based on age-adjusted reference ranges 4, 5
• For patients >80 years: TSH values up to 8.0 mIU/L may be acceptable without adverse effects on quality of life or cardiovascular risk 3
• Avoid overtreatment: Elderly patients are particularly susceptible to atrial fibrillation from excessive thyroid hormone 1

Practical Considerations:

• Start levothyroxine at lower doses (50 µg daily for those >50-60 years without coronary disease, or 12.5-25 µg daily with known cardiac disease) 1
• Elderly patients require 20-25% less levothyroxine per kilogram than younger patients 1
• In patients >80-85 years with TSH ≤10 mIU/L, consider a "watch and wait" approach rather than immediate treatment 4

Special Populations

Subclinical Hypothyroidism:

• TSH >10 mIU/L: Treat to normalize TSH into reference range 1
• TSH 4.12-10 mIU/L: Consider treatment based on symptoms, positive TPO antibodies, or cardiovascular risk factors; target normal TSH range if treating 1

Central Hypothyroidism:

• Do not use TSH for monitoring 1
• Target free T4 in the upper half of the normal reference range 1
• Measure free T4 before taking levothyroxine dose 1

Common Pitfalls to Avoid

1. Over-replacement is common (20% of treated patients) and increases cardiovascular and skeletal risks 1
2. Do not target TSH <0.4 mIU/L unless treating thyroid cancer (beyond scope of these guidelines) 1
3. Pregnancy targets are non-negotiable - failure to achieve them risks fetal neurodevelopmental complications 6
4. Age matters - using the same TSH target for an 85-year-old as a 35-year-old leads to overtreatment 4, 3, 5