Free T4
Measures the thyroxine (T4) that is not stuck to blood proteins, so it can actually enter your cells. Doctors pair it with TSH when the thyroid story does not add up from TSH alone or when dose changes need a second number.
Thyroxine is the main hormone your thyroid gland ships out; only the free fraction can slip into tissues and steer metabolism. TSH is still the usual first screen, but free T4 fills the gap when TSH sits in a gray zone, symptoms do not match the number, or you are tuning levothyroxine after a dose change. Pregnancy, pituitary disease, and some medicines rewrite the rules, so your doctor reads free T4 next to history and exam. One lab value is a waypoint, not the whole map.
Self-Pay Price Comparison
| Provider | Price | vs. Highest | Order |
|---|---|---|---|
Quest QuestHealth self-pay | Not Available | N/A | Check Provider Site · Quest |
LabCorp Labcorp OnDemand | Not Available | N/A | Check Provider Site · LabCorp |
GoodLabs Discount lab network | $6.00Best value | Best price | Order · GoodLabs |
What This Test Measures
Free T4 is reported as a single concentration, usually ng/dL or pmol/L depending on the lab. Here is what that free fraction represents:
Thyroid hormone that is not bound to carrier proteins so it can enter cells; low values fit an under-producing gland or odd pituitary signaling, high values fit an overactive gland or too much replacement hormone.
How to Read Your Results
Reference intervals move with age, pregnancy, and assay. Here is how to read the three pattern rows doctors use after TSH:
| Marker | Normal Range | If Flagged | What It Might Mean |
|---|---|---|---|
| Free T4 (adult, general) | MedlinePlus Medical Encyclopedia, thyroid function tests: reference intervals for free thyroxine depend on age, pregnancy, and laboratory method; use the range printed on your report. | abnormal | Slightly high or low free T4 with normal TSH can be lab variation, timing, or biotin interference. MedlinePlus warns high-dose biotin can skew thyroid immunoassays. One result in isolation rarely tells the full story. |
| Free T4 (low with high TSH) | Pattern context per MedlinePlus Thyroxine (T4) Test: primary hypothyroidism often pairs higher TSH with lower free T4. | low | Usually points toward an underactive thyroid gland, but pituitary problems can create odd pairings. Your doctor confirms with repeat labs and symptoms before locking a plan. |
| Free T4 (high with low TSH) | Pattern context per MedlinePlus Thyroxine (T4) Test: hyperthyroidism often pairs lower TSH with higher free T4. | high | Fits an overactive thyroid or too much thyroid hormone, yet stress illness and some drugs mimic pieces of the pattern. Imaging or antibody tests may follow. |
When to Order
Annual baseline
Most people do not need yearly free T4 by itself; it shows up when thyroid symptoms, abnormal TSH, or treatment monitoring is already on the table.
Abnormal or changing TSH
When TSH is off or bouncing, free T4 shows whether the thyroid is actually under- or over-producing hormone.
Starting or adjusting levothyroxine
Repeat free T4 with TSH helps your clinician see whether the new dose landed in a safe window.
Hyperthyroid symptoms (racing heart, weight loss, tremor)
Low TSH plus high free T4 fits an overactive thyroid pattern; imaging or antibodies may come next.
Pregnancy or planning pregnancy
Thyroid targets tighten in pregnancy; obstetric and endocrine teams often track free T4 with TSH by trimester.
Prep & Logistics
- Fasting
- Typically no fasting
- Sample
- Blood draw
- Results
- Usually 24-48 hours; many portals update the same day or the next.
- Referral
- Often self-order (check local rules)
- Markers
- Free T4 (often ng/dL or pmol/L depending on the lab).
Tests That Pair With This One
FAQ
Why order Free T4 if my TSH already looks normal?
Subclinical patterns, pregnancy, pituitary disease, and some medicines break the usual TSH-first story. Free T4 is the hormone fraction tissues actually see; it helps when symptoms and TSH disagree.
My TSH is normal but I still feel awful. Should I insist on Free T4?
Sometimes. When symptoms and TSH disagree, free T4 (and sometimes free T3) helps break the tie. Other issues like anemia, sleep apnea, or depression can mimic thyroid trouble, so your doctor may cast a wider net.
Does biotin shampoo matter?
Unlikely. MedlinePlus focuses on high-dose biotin pills skewing blood immunoassays. Tell the lab and your doctor what you take; they may ask you to pause supplements before the draw.
Why is Free T4 different from total T4?
Total T4 counts hormone stuck to proteins plus the free fraction. Protein levels from pregnancy or birth control pills can move total T4 while free T4 stays steady, which is why clinicians often prefer the free test.
Sources
Prices pulled directly from provider websites and verified by hand. Reference ranges sourced from MedlinePlus. Not generated by AI.
- GoodLabs - product page (pricing context)(verified 2026-03-23)
- Quest - consumer lab shop (pricing context)(verified 2026-03-23)
- LabCorp OnDemand - consumer lab catalog (pricing context)(verified 2026-03-23)
- MedlinePlus - Thyroxine (T4) Test overview
- MedlinePlus Medical Encyclopedia - Thyroid function tests
- Clinical context: LabRecon editorial team. Not medical advice. For informational use only.
Immunoassay for unbound thyroxine (T4). Used with TSH to differentiate primary from secondary thyroid dysfunction and guide levothyroxine dosing.
Ordering note
Phase 3 backlog: add Quest/LabCorp standalone ft4 SKU URLs and pricing when scraped.