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Serum or plasma concentration of active (bioactive) thyroid hormone (T4) measured via immunoassay

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

This test measures the amount of the active form of thyroid hormone (T4) in your blood, which is important for regulating metabolism and energy levels.

Quick summary

["Assesses your thyroid function and helps diagnose or monitor thyroid disorders."]

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

Key facts at a glance

LOINC
50072-8
Short code
11DC pre 250 ug ACTH SerPl-sCnc
Synonyms
11DC pre 250 ug ACTH SerPl-sCnc
Category
hormone
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
20
Ref high
400
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
82670
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 Is This Test Done?

Your healthcare provider may order this test if you have symptoms like fatigue, weight changes, or unexplained goiter (thyroid enlargement). It’s also used to monitor thyroid hormone levels in conditions like hypothyroidism or hyperthyroidism.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before the draw if your lab requires it. Wear a loose shirt for easy access to your arm.

What Happens During the Test?

A healthcare professional will insert a small needle into your arm to draw blood. You may feel a brief pinch, but it usually doesn’t cause discomfort. The blood sample is then sent to the lab for analysis.

What Does the Test Result Mean?

Results help determine if your thyroid is functioning normally. Abnormal levels may indicate conditions like hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). Your provider will discuss these findings with you.

Reference range

Premenopausal: 20-400 pg/mL; Postmenopausal: <30 pg/mLN/A

Specimen: serum

Typically collected in the morning (8-10 AM). Inform your provider if taking steroid medications.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Free T4 (Serum/Plasma)

Frequently asked questions

Why might my T4 levels be checked regularly?
If you have a history of thyroid disorders, take thyroid medication, or have symptoms like unexplained weight changes or fatigue, your provider may monitor your T4 levels to ensure they’re stable and effective.
What if my T4 levels are too high or too low?
High levels may suggest hyperthyroidism, which can cause rapid heartbeat, anxiety, or weight loss. Low levels may indicate hypothyroidism, leading to fatigue, weight gain, or depression. Treatment (like medication or thyroid hormone replacement) may be needed to balance them.
Does diet affect T4 test results?
Certain foods (like soy, goitrogens) or supplements (like selenium) can temporarily affect thyroid function. Avoiding these for a few days before testing may help get more accurate 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