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Thyroid Hormone Free T4 (Serum) with Thyroid-Stimulating Hormone (TSH) Ratio

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

This test measures the ratio of free thyroid hormone T4 to thyroid-stimulating hormone (TSH) in your blood to help assess thyroid function.

Quick summary

["This ratio helps doctors evaluate whether your thyroid is producing enough hormone or if it’s being overstimulated."]

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

Key facts at a glance

LOINC
42300-4
Short code
MR Thyroid
Synonyms
MR Thyroid
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
mr
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 doctor may order this test if you have symptoms like fatigue, weight changes, or unexplained mood swings that could relate to thyroid issues. It’s also used to monitor thyroid conditions like hypothyroidism or hyperthyroidism, especially when other tests show conflicting results.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before the blood draw if your lab uses a specific method. Wear comfortable clothing and plan to stay near the testing site for a short time.

How the Test Works

Blood is drawn from a vein in your arm. The sample is sent to a lab where it’s analyzed to measure free T4 and TSH levels. The ratio helps determine if your thyroid is functioning normally or if hormone imbalances exist.

What the Results Mean

Results are interpreted by your healthcare provider based on your symptoms and other test results. A ratio that’s too high or too low may signal thyroid dysfunction, prompting further evaluation.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Free T4:TSH Ratio Test

Frequently asked questions

What does a high or low ratio indicate?
A high ratio might suggest hyperthyroidism (overactive thyroid), while a low ratio could indicate hypothyroidism (underactive thyroid). Your doctor will use these results alongside other factors to guide diagnosis and treatment.
Can diet affect this test?
Dietary changes like excessive iodine intake (e.g., seafood, supplements) or thyroid medications can influence thyroid hormone levels. Avoiding these temporarily may help ensure accurate results.
How often should I get this test?
Frequency depends on your health status. If you have a thyroid condition, your doctor may recommend regular monitoring to track changes in your hormone levels.
  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