Low free t4

Written to our editorial standards · reviewed against published research· Updated 2 Jun 2026
A medical finding, review with a doctor

A low free T4, usually alongside a high TSH, points to an underactive thyroid and the same symptoms (fatigue, cold, weight gain), and is a medical finding for your doctor.

Free T4 is the available thyroid hormone, so a low level confirms the thyroid is not producing enough, typically with a raised TSH. Selenium supports thyroid enzymes, but treatment of an underactive thyroid is medical.

Common causes

  • Autoimmune thyroiditis (Hashimoto's)
  • Iodine deficiency
  • Pituitary problems (rarely)
  • Previous thyroid treatment

Associated symptoms

  • Fatigue
  • Feeling cold
  • Weight gain
  • Dry skin
  • Low mood

Nutrients that can help

Selenium
See the guide →
What to do

Take this to your doctor with your TSH. Selenium and adequate iodine support the thyroid, but a low free T4 is managed medically.

When to see a doctor

A low free T4 needs a doctor's review and usually treatment with monitoring.

Full marker
Free T4: ranges, what it measures & both directions
Read the Free T4 guide →

Common questions

What does low free t4 mean?
A low free T4, usually alongside a high TSH, points to an underactive thyroid and the same symptoms (fatigue, cold, weight gain), and is a medical finding for your doctor. Free T4 is the available thyroid hormone, so a low level confirms the thyroid is not producing enough, typically with a raised TSH. Selenium supports thyroid enzymes, but treatment of an underactive thyroid is medical.
What causes low free t4?
Autoimmune thyroiditis (Hashimoto's); Iodine deficiency; Pituitary problems (rarely); Previous thyroid treatment.
How do I raise my free t4?
Take this to your doctor with your TSH. Selenium and adequate iodine support the thyroid, but a low free T4 is managed medically.
When should I see a doctor?
A low free T4 needs a doctor's review and usually treatment with monitoring.

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Educational use only, not medical advice or diagnosis. Reference ranges vary by lab, age, sex, and medication, and a single result is read in context. Always review your results with a qualified clinician.