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Thyroid Stimulating Hormone (TSH) – Serum

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

This blood test measures your thyroid-stimulating hormone, a key hormone that regulates your thyroid function and metabolism.

Quick summary

["TSH helps doctors check if your thyroid is working properly and adjust treatment if needed."]

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

Key facts at a glance

LOINC
97166-3
Short code
T4 24h Ur-mRate
Synonyms
T4 24h Ur-mRate
Category
chemistry
Sample
urine
Container
Sterile urine container
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
10 mL
Stability
Room temperature: 2h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your doctor may order a TSH test to diagnose or monitor thyroid conditions, such as hypothyroidism (low thyroid activity) or hyperthyroidism (overactive thyroid). It’s often the first test used to check thyroid function because it’s sensitive and easy to measure.

What to Expect Before the Test

No special preparation is usually needed, but you may be asked to fast for 8–12 hours before the test if your lab uses a different method. Wear comfortable clothing for blood draws.

How the Test Works

A small needle is inserted into your arm to draw blood. The sample is sent to a lab, where it’s tested for TSH levels using an immunoassay method to detect the hormone in your serum.

What the Results Mean

Results are reported in international units per liter (IU/L). High or low TSH levels can signal thyroid issues, but your doctor will interpret results in the context of your symptoms and other tests.

Reference range

Adult: 4.5-12.0 μg/dLN/A

Specimen: urine

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.

TSH is crucial because it acts as a feedback mechanism: if your thyroid isn’t producing enough hormone, TSH rises to stimulate production; if it’s overactive, TSH drops. Abnormal levels often prompt further testing for thyroid disease, including ultrasound or additional hormone checks.

Short name

TSH

Frequently asked questions

What causes TSH levels to be too high?
High TSH is most commonly linked to hypothyroidism (underactive thyroid), but it can also result from pituitary disorders, thyroid surgery, or certain medications. Rarely, it may indicate thyroid cancer or other conditions.
What causes TSH levels to be too low?
Low TSH often signals hyperthyroidism (overactive thyroid), but it can also occur with pituitary tumors, thyroiditis (inflammation), or thyroid hormone resistance. Stress, pregnancy, or thyroid nodules may also play a role.
Can TSH levels change over time?
Yes, TSH levels can fluctuate due to age, medications (like thyroid replacement drugs), stress, or underlying health conditions. Regular monitoring may be needed if you have a thyroid disorder or take thyroid medication.
  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