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Thyroid Hormone, Total (Serum)

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

This test measures the total amount of thyroxine (T4), a key hormone produced by your thyroid gland. It helps assess thyroid function and whether your body is making enough thyroid hormone.

Quick summary

["Total T4 levels help diagnose and monitor thyroid disorders like hypothyroidism or hyperthyroidism."]

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

Key facts at a glance

LOINC
108777-4
Short code
T4BG Msmt SerPl
Synonyms
T4BG Msmt SerPl
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 Matters

Your thyroid gland releases thyroid hormones, including T4, which regulate metabolism, energy levels, and other bodily functions. If your levels are too high or too low, it can cause symptoms like fatigue, weight changes, or mood swings. This test helps doctors check if your thyroid is working properly.

What to Expect Before the Test

No special preparation is needed for this blood test. You may have a small needle inserted into your arm to draw blood. If you’re on thyroid medication, let your healthcare provider know before the test.

Common Symptoms Linked to Results

Low total T4 might indicate hypothyroidism (underactive thyroid), leading to fatigue, weight gain, or cold sensitivity. High total T4 could suggest hyperthyroidism (overactive thyroid), causing weight loss, anxiety, or rapid heartbeat. Your doctor will interpret results in relation to your symptoms and other tests.

How Results Are Used

Your doctor will compare your total T4 level to reference ranges and use it alongside other tests (like TSH) to diagnose or manage thyroid conditions. They may also check it periodically if you have a thyroid disorder or take thyroid medication.

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.

Clinical Significance: Total T4 Levels

Short name

Total T4

Frequently asked questions

What causes false results in this test?
False results can occur if you have recent thyroid medication, pregnancy, or certain medications (like steroids or amiodarone). Blood samples taken shortly after exercise or stress may also affect accuracy. Always follow your doctor’s instructions for timing and preparation.
How often should I get this test?
The frequency depends on your health. If you have no thyroid issues, routine checks may not be needed unless advised by your doctor. Those with thyroid disorders may need monitoring every few months or annually, depending on treatment.
Can I eat or drink before this test?
You can eat and drink normally before this test, as no fasting is required. However, avoid caffeine or alcohol in the hour before your appointment, as they might slightly affect blood test results.
  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