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Thyroid Stimulating Hormone (TSH) by Immunoassay (Serum/Plasma)

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

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

Quick summary

["TSH helps doctors check if your thyroid is working properly."]

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

Key facts at a glance

LOINC
1642-8
Short code
TSH BS SerPl-mCnc
Synonyms
TSH BS SerPl-mCnc
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

What Is TSH Used For?

TSH is primarily used to diagnose and monitor thyroid disorders, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It can also help determine if you have thyroid cancer or other conditions affecting thyroid hormone levels.

How Is the Test Performed?

A blood sample is taken from a vein, usually in your arm. The sample is sent to a lab where it is analyzed using an immunoassay method to measure TSH levels.

What Does a TSH Test Tell You?

Elevated TSH levels may indicate hypothyroidism, while very low levels could suggest hyperthyroidism or other thyroid issues. Normal results suggest your thyroid is functioning within a typical range, though additional tests may be needed for further evaluation.

Common Reasons for Testing

Your doctor may order a TSH test if you have symptoms like fatigue, weight changes, or unexplained issues with your metabolism. It’s also used as part of routine check-ups or before certain treatments.

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

Frequently asked questions

Why might my TSH test results be different from what I expected?
Several factors can influence TSH levels, including medications (like thyroid hormone replacement), pregnancy, stress, or certain medical conditions. Your doctor will discuss these with you based on your full medical history.
What should I do if my TSH test is abnormal?
If your TSH results are outside the expected range, your doctor may recommend further testing, such as checking free T4 or T3 levels, or other evaluations to determine the cause and guide treatment options.
Can I eat or drink anything before my TSH test?
You can typically eat and drink normally before your TSH test unless your doctor specifies otherwise. Avoiding certain medications or supplements may also be advised, so follow your healthcare provider’s instructions.
  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