Skip to content

Thyroid hormone total serum concentration (T4)

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of thyroxine (T4), a hormone produced by your thyroid gland that helps regulate metabolism.

Quick summary

["Your doctor may order this test to check thyroid function or monitor treatment for thyroid disorders."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
35226-0
Short code
T4 SerPl-msCnc
Synonyms
T4 SerPl-msCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
4.5
Ref high
12
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
84436
SNOMED
302345006
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your doctor might recommend a total T4 test if you have symptoms like fatigue, weight changes, or unexplained hair loss. It’s also used to diagnose or monitor conditions such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid).

What to Expect Before the Test

No special preparation is needed for this blood test. You may be asked to fast for 8–12 hours before the draw, but your doctor will confirm this if it’s part of a larger thyroid panel.

How the Test Works

A small amount of blood is drawn from a vein in your arm. The blood sample is sent to a lab where it’s analyzed to measure the total T4 concentration in your serum.

What the Results Mean

Results alone don’t provide a full picture of thyroid health. They’re usually interpreted alongside other thyroid tests (like TSH) and clinical symptoms to diagnose conditions accurately.

Reference range

Adult: 4.5-12.0 μg/dLN/A

Specimen: serum

Take your thyroid medication after the blood draw if applicable. No fasting required.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Total Serum Thyroxine (T4)

Frequently asked questions

What could affect my T4 test results?
Several factors can influence T4 levels, including medications (like thyroid hormone replacement), pregnancy, stress, or certain medications (e.g., steroids). If you’re on long-term medication, discuss any changes with your doctor before retesting.
Do I need to take any special steps after the test?
No special steps are needed after the test. However, if you experience bruising, swelling, or pain at the injection site, apply gentle pressure and elevate the arm. Contact your doctor if symptoms worsen or you notice unusual bleeding.
Can I get my T4 test results over the phone?
Results are typically reviewed by your healthcare provider and shared with you during a follow-up visit. Some labs may offer digital access to preliminary results, but confirm with your doctor or lab for details.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy