Free T3 (triiodothyronine)

Pick Ordered or Not ordered to see the note.

Indicated when

• —Suspected secondary (pituitary) hypothyroidism with a low TSH, measured in the same sample as TSH and free T4 [1]
• —Suspected T3 toxicosis: hyperthyroid symptoms with a suppressed TSH but a normal free T4, to look for isolated T3 elevation [2]

Not indicated when

• —Screening for hypothyroidism [3]
• —Monitoring or adjusting levothyroxine dose in a patient with known primary hypothyroidism [3]
• —Investigating fatigue, low mood, or weight concerns without an abnormal TSH [College of Family Physicians of Canada / Choosing Wisely Canada 2026, 5]

Why not

T3 levels are strongly affected by non-thyroidal illness and by conversion from T4 in peripheral tissue, so an isolated free T3 without an abnormal TSH is hard to interpret and rarely changes management. Choosing Wisely Canada's endocrinology recommendation specifically names free T3 as a test not to use for screening or for adjusting levothyroxine dose [3], and the American Thyroid Association's hypothyroidism treatment guideline found no consistently strong evidence that adding T3 therapy improves outcomes over standard levothyroxine treatment monitored by TSH [4].

Better first step

Start with TSH; add free T4 if abnormal. Reserve free T3 for a suppressed TSH with a normal free T4 (possible T3 toxicosis) or for suspected secondary hypothyroidism alongside TSH and free T4 [1, 2].

Typical ND rationale

An ND may order free T3 because T3 is the more biologically active thyroid hormone, reasoning that a person can feel hypothyroid with fatigue, weight gain, or low mood even when TSH and free T4 look normal, and that measuring T3 directly checks whether enough of it is being made or converted.

Ontario coverage & CONO orderability

OHIP status

insured

Free T3 is listed by name (L607, 'T-3, free') in the 2026 Schedule with a standard professional fee ($3.53), so it is insured when a physician who has clinically assessed the patient orders it. ND-ordered testing is patient-paid regardless (Reg. 552 s.22).

CONO orderable

Yes — CONO list #126

CONO list item 126, blood: 'Triiodothyronine Free (T3).'

Linked conditions

• Hypothyroidism ↗
• "Low T3" / reverse T3 dominance ↗

Counselling script

“Free T3 is a test with a couple of narrow, specific uses, not a routine part of thyroid testing. When the TSH is normal, a free T3 result, even a low one, wouldn't change what I'd do. If your TSH is ever suppressed with a normal free T4, or if I suspect a pituitary cause, that's when free T3 becomes useful and I'll order it.”

Revisit if

• —TSH becomes suppressed with a normal free T4
• —New signs of hyperthyroidism: tremor, palpitations, heat intolerance, unintentional weight loss
• —New symptoms suggesting pituitary disease

References

1. [1]National Institute for Health and Care Excellence (NICE) (2019). Thyroid disease: assessment and management (NG145). linkFree T3 is measured alongside TSH and free T4 when TSH is below the reference range, and when secondary hypothyroidism is suspected
2. [2]Ontario Association of Medical Laboratories (OAML) (2007). Guideline for the Use of Laboratory Tests to Detect Thyroid Dysfunction (CLP 015). linkolder guidelineFree thyroid hormone levels (including free T3) are used to confirm and further investigate an abnormal TSH, not as a stand-alone screen
3. [3]Canadian Society of Endocrinology and Metabolism / Choosing Wisely Canada (2026). Endocrinology and Metabolism: Seven Tests and Treatments to Question. linkDon't use free T4 or T3 to screen for hypothyroidism or to monitor and adjust levothyroxine dose in patients with known primary hypothyroidism
4. [4]American Thyroid Association (2014). Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. linkolder guidelineNo consistently strong evidence that T3-based approaches are superior to standard levothyroxine therapy and TSH-based monitoring
5. [5]American Association of Clinical Endocrinologists / American Thyroid Association (2012). Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. linkolder guidelineTSH is the single best screening test for primary thyroid dysfunction, not free T3