TSH, T3, and T4

What it is

TSH, T3, and T4 are the three hormones measured in a standard thyroid panel — the most commonly ordered endocrine blood test in Canada, where thyroid disorders affect an estimated 1 in 10 Canadians, with women diagnosed roughly 5 to 8 times more often than men. TSH (thyroid-stimulating hormone) is produced by the pituitary gland and acts as the master signal that tells the thyroid how much hormone to make. T4 (thyroxine) and T3 (triiodothyronine) are the two hormones the thyroid actually secretes; T4 is the more abundant form and converts to the more metabolically active T3 in peripheral tissues.

Together, these three markers form what clinicians call a thyroid function test (TFT) or thyroid panel. In Canada, panels are processed through national laboratory networks including LifeLabs and Dynacare, and are covered under provincial health insurance when ordered by a licensed clinician.

Causes and mechanism

The three hormones operate through a feedback loop. When T3 and T4 levels fall, the hypothalamus releases thyrotropin-releasing hormone (TRH), which prompts the pituitary to secrete more TSH. Rising TSH then stimulates the thyroid to produce more T4 and T3. When levels are adequate, TSH production is suppressed. This negative feedback means TSH moves in the opposite direction to T4 and T3 — a pattern that is central to interpreting results.

Because T4 and T3 circulate partly bound to proteins, laboratories typically report the unbound fractions: free T4 (fT4) and free T3 (fT3). These free fractions are biologically active and more clinically meaningful than total hormone levels, particularly in pregnancy or when a patient is taking oral contraceptives, which raise binding proteins and can inflate total T4 without reflecting true thyroid status.

Symptoms and diagnosis

Thyroid dysfunction produces a wide range of symptoms depending on whether hormone levels are too high or too low.

Pattern	TSH	fT4	fT3	Likely diagnosis
High TSH, low fT4	↑	↓	↓ or normal	Primary hypothyroidism
Low TSH, high fT4/fT3	↓	↑	↑	Primary hyperthyroidism
Low TSH, normal fT4/fT3	↓	Normal	Normal	Subclinical hyperthyroidism
High TSH, normal fT4	↑	Normal	Normal	Subclinical hypothyroidism
Low TSH, low fT4	↓	↓	↓	Central (pituitary) hypothyroidism

According to a widely cited NIH review, TSH is the single most useful test of thyroid function for the vast majority of patients, and primary care providers rarely need to order T3 or T4 without an abnormal TSH result first. Reference ranges vary by laboratory and age; most Canadian labs report a TSH reference interval of approximately 0.4–4.0 mIU/L, though this narrows in older adults and shifts during pregnancy.

Symptoms of hypothyroidism include fatigue, weight gain, cold intolerance, constipation, and depression. Hyperthyroidism typically presents with weight loss, palpitations, heat intolerance, tremor, and anxiety. Because these symptoms overlap with many other conditions, blood testing is essential for diagnosis.

Treatment options

Treatment depends entirely on the underlying diagnosis, not on the test values alone.

• Hypothyroidism is treated with levothyroxine (synthetic T4), titrated to normalize TSH. Most Canadian provincial formularies cover levothyroxine as a first-line drug benefit.
• Hyperthyroidism may be managed with antithyroid medications (methimazole, propylthiouracil), radioactive iodine ablation, or thyroidectomy, depending on cause and severity.
• Subclinical hypothyroidism (elevated TSH with normal fT4) is managed case-by-case; treatment thresholds and benefit are debated, particularly in older adults.
• Subclinical hyperthyroidism (suppressed TSH with normal fT4/fT3) is monitored closely given cardiovascular and bone-density risks.

Canadians seeking thyroid assessment outside of a traditional GP referral can access requisitions through virtual care platforms such as Maple, Felix, or Cleo, though follow-up interpretation and prescribing still require a licensed clinician.

When to see a clinician in Canada

Request a thyroid panel from your family physician or nurse practitioner if you have unexplained fatigue, significant weight change, mood disturbance, or a family history of thyroid disease. Thyroid testing is also recommended during pregnancy planning and in the first trimester, as both hypothyroidism and hyperthyroidism carry fetal risks. The Society of Obstetricians and Gynaecologists of Canada (SOGC) supports targeted thyroid screening in high-risk pregnant patients. If your TSH is abnormal, your clinician will typically order fT4 and possibly fT3 before referring to an endocrinologist.

Limitations and open questions

Research is still emerging on the optimal TSH reference range for older adults, with some evidence suggesting that a TSH up to 6–7 mIU/L may be normal in patients over 70. Health Canada has not issued a unified national reference range, so values flagged as abnormal can differ between LifeLabs and Dynacare depending on the assay used. The clinical benefit of treating subclinical hypothyroidism — particularly in patients over 65 — remains contested; a 2017 randomized trial published in the New England Journal of Medicine found no symptomatic benefit from levothyroxine in this group. The role of routine fT3 measurement in monitoring patients on levothyroxine therapy is also debated, as some patients report persistent symptoms despite normal TSH and fT4, and whether adding T3 (liothyronine) improves outcomes is not yet established by high-quality evidence. Clinicians and patients should interpret borderline results in the context of symptoms, not numbers alone.