Free T4 (thyroxine)

Pick Ordered or Not ordered to see the note.

Indicated when

• —Confirming and characterizing an abnormal TSH, in the same sample as the repeat TSH [1, 2]
• —Suspected secondary (pituitary or hypothalamic) hypothyroidism, ordered together with TSH, because TSH itself becomes unreliable in that situation [1]
• —Monitoring and adjusting therapy in a patient with confirmed secondary hypothyroidism, where TSH cannot be used to guide dosing [1]

Not indicated when

• —As a stand-alone screening test without a TSH [2, 1]
• —Routine monitoring or levothyroxine dose adjustment in a patient with known primary hypothyroidism: current Canadian guidance is to use TSH alone for this [3]

Why not

Free T4 can be affected by non-thyroidal illness, pregnancy, and assay differences between labs, so an isolated result without a TSH for context is harder to interpret. In known primary hypothyroidism, TSH already reflects the pituitary's fine-grained response to circulating thyroid hormone, so current Canadian guidance holds that adding free T4 to routine monitoring doesn't change management, only cost and the chance of a discordant, confusing result [3].

Better first step

Order TSH first; add free T4 to the same sample only if the TSH is abnormal, or if secondary (pituitary) disease is suspected, in which case order both together from the start [1, 2].

Typical ND rationale

An ND commonly includes free T4 alongside TSH from the start, reasoning that TSH alone can miss secondary (pituitary) hypothyroidism and that seeing the actual circulating hormone level gives a fuller picture than TSH alone.

Ontario coverage & CONO orderability

OHIP status

unverified

Free T4 is listed by name in the 2026 Schedule (L339, 'T-4, free absolute'), but unlike TSH (L341, $3.58) and free T3 (L607, $3.53), it carries a listed professional fee of $0.00. What that $0.00 fee means for physician-ordered billing (bundled into another code, technical-fee-only, or otherwise) could not be established from the Schedule text itself, so coverage is recorded as unverified rather than assumed insured. ND-ordered testing is patient-paid regardless (Reg. 552 s.22).

CONO orderable

Yes — CONO list #120

CONO list item 120, blood: 'Thyroxine Free (FT4).'

Linked conditions

• Hypothyroidism ↗

Counselling script

“Free T4 is a follow-up test, not a stand-alone one, so I order it together with TSH rather than by itself. If your TSH is normal and there are no signs of a pituitary problem, adding free T4 doesn't tell us anything more. If your TSH does come back abnormal, or if I ever suspect a pituitary cause, I'll order free T4 in the same sample right away.”

Revisit if

• —TSH result comes back abnormal
• —New symptoms suggesting pituitary disease (headache, visual field change, other hormone symptoms)
• —Known secondary hypothyroidism with a change in symptoms or medication dose

References

1. [1]National Institute for Health and Care Excellence (NICE) (2019). Thyroid disease: assessment and management (NG145). linkFree T4 is measured alongside TSH to confirm an abnormal result, and TSH plus FT4 are ordered together 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 guidelinesTSH alone is the appropriate initial test; free thyroid hormone levels confirm and further investigate an abnormal result
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