TSH & free T4
Also known as:
Thyroid-stimulating hormone, thyroxine, thyroid panel
The core thyroid tests, how to read them, and when the numbers can mislead.
Written and reviewed by Farhan Tahir, MD · Last reviewed
VIDEO LESSON
What they measure
TSH is the pituitary's signal to the thyroid. Free T4 is the unbound thyroid hormone available to tissues. Because the pituitary responds strongly to small changes in T4, TSH is the most sensitive first test.
Reading the results
• High TSH, low free T4: overt hypothyroidism
• High TSH, normal free T4: subclinical hypothyroidism
• Low TSH, high free T4 or T3: hyperthyroidism
• Low TSH with low free T4: possible pituitary (central) disease, where TSH is unreliable
On levothyroxine
Recheck TSH 6 to 8 weeks after a dose change. Take the dose consistently and keep it apart from calcium, iron and some antacids.
Good to know
• TSH naturally drifts higher with age.
• Acute illness, pregnancy and some medicines shift results.
• High-dose biotin can produce falsely abnormal thyroid tests.
Key studies and references
Educational content only, written and reviewed by Farhan Tahir, MD. It does not replace individual medical advice. For sudden chest pain, trouble breathing or stroke symptoms, call 911.
AT A GLANCE
Test type
Thyroid
Also known as
Thyroid-stimulating hormone, thyroxine, thyroid panel
Reviewed by
Farhan Tahir, MD
Questions about your own care?
Dr. Tahir’s team offers consults through Youthful Medicine.
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