Free T4 · low result
What does a low Free T4 mean?
Short answer
Low Free T4 with a high TSH is overt hypothyroidism — the thyroid isn't making enough hormone. Low Free T4 with a low or normal TSH suggests the pituitary or hypothalamus isn't signalling properly (central hypothyroidism) or that a severe non-thyroidal illness is suppressing the axis. Recovery from illness and certain medicines also lower it.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
What is the normal range for Free T4?
- age 18+: 0.820 – 1.77 ng/dL
Usually reflects an underactive thyroid or pituitary issues.
Full reference table, citations and testing options: Free T4 biomarker page.
What are the common causes of low Free T4?
- Primary hypothyroidism (Hashimoto's, post-surgery, post-radioiodine)
- Under-replacement on levothyroxine or missed doses
- Central hypothyroidism from a pituitary or hypothalamic problem
- Severe non-thyroidal illness
- Medications such as phenytoin, carbamazepine or high-dose glucocorticoids
What symptoms go with low Free T4?
- Fatigue, cold intolerance, weight gain
- Dry skin, hair loss, constipation
- Slowed thinking and low mood
What do clinicians usually check next?
- Interpret alongside TSH — the pair defines the cause
- TPO antibodies for autoimmune thyroid disease
- Morning cortisol and other pituitary hormones when TSH is inappropriately normal or low
- Repeat after 6–8 weeks of any dose change
Follow-up tests available at TestWell: TSH (Thyroid Stimulating Hormone) ($26.99), Thyroid Peroxidase (TPO) Antibodies ($25.99), Cortisol (AM) ($26.99), Cortisol ($32.99).
How do you recheck Free T4?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Complete Thyroid Panel | Panel · 4 tests | $109.99 | No | 3–7 business days |
| Women's Health Panel | Panel · 8 tests | $129.99 | Yes | 2–5 business days |
| Pre-Pregnancy Panel | Panel · 8 tests | $224.99 | Yes | 1–3 business days |
| TSH + Free T4 | Test · 2 biomarkers | $27.99 | No | 1–3 business days |
Common questions
Can Free T4 be low with a normal TSH?
Yes, and it deserves attention: it can indicate a pituitary problem rather than a thyroid one. Clinicians usually repeat the test and check other pituitary hormones.
Sources
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.