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Thyroid Stimulating Hormone (TSH) with Retrospective Measurement of Free T4 (Free Thyroxine)

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

This test measures your thyroid-stimulating hormone (TSH) level and evaluates free thyroxine (a form of thyroid hormone) to assess thyroid function and hormone balance.

Quick summary

["This test helps diagnose and monitor thyroid disorders, including hypothyroidism and hyperthyroidism."]

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

Key facts at a glance

LOINC
25991-1
Short code
TSH pre TRH IN SerPl-sCnc
Synonyms
TSH pre TRH IN SerPl-sCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
0.4
Ref high
4
Critical range
0.1 N/A – 20 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
84443
SNOMED
301343003
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 healthcare provider may order this test if you have symptoms like fatigue, weight changes, or unexplained mood swings. It’s also used to check thyroid function before starting thyroid hormone treatment or during follow-ups for thyroid conditions.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before drawing blood. Avoid smoking or caffeine 30 minutes before your visit.

What Happens During the Test

A healthcare professional will insert a needle into your arm to draw blood. The process usually takes a few minutes, and you may feel a brief pinch. The sample is then sent to a lab for analysis.

What the Results Mean

Results show two key measurements: TSH (in mIU/L) and free T4 (in pmol/L). Abnormal results may indicate thyroid dysfunction, which your provider will discuss with you.

Reference range

Adult: 0.4-4.0 mIU/LN/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

TSH + Free T4 Profile

Frequently asked questions

What does a high or low TSH level mean?
A high TSH level often suggests hypothyroidism (underactive thyroid), while a low TSH level may indicate hyperthyroidism (overactive thyroid) or other pituitary issues. Free T4 helps confirm whether your thyroid hormone levels are normal or affected.
How often should I get this test?
Your provider will determine frequency based on your symptoms, medical history, and treatment needs. Regular follow-ups may be recommended if you have a thyroid condition.
Can diet or supplements affect these test results?
Yes, certain foods (like soy or iodine-rich foods) or supplements (e.g., selenium) may influence thyroid function. Avoid them for a few days before testing to get 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