What does it measure?
The pituitary gland releases thyroid-stimulating hormone to tell the thyroid how much hormone to make. When thyroid hormone supply falls, TSH often rises to ask for more. The test measures this feedback signal rather than thyroid hormone itself. [1]
Before the test
- Sample
- A blood sample from a vein.
- Supplements
- Mention biotin and thyroid medicines; follow the laboratory’s preparation advice for its assay.
What can the result mean?
High TSH can suggest an underactive thyroid, while low TSH can suggest excess thyroid hormone. The free T4 result helps tell whether thyroid hormone supply is actually low, normal, or high. Pituitary conditions, serious illness, and some medicines can produce different patterns. [1]
When is it useful, and how is it read?
TSH and free T4 together help distinguish different thyroid patterns. High TSH with low free T4 points toward primary hypothyroidism, while low TSH with low free T4 calls for a different assessment that may involve the pituitary, illness, or medicines. [2] [3]
Reading the results together
| Marker | What it adds |
|---|---|
| Thyroid Stimulating Hormone (TSH) | The pituitary’s signal to the thyroid. |
| Free T4 | The unbound T4 available in blood. |
A raised TSH means something different depending on whether free T4 is low or within range. Pregnancy, medicines, and pituitary conditions can change interpretation. [2]
What can affect it?
Age, pregnancy, recent illness, and thyroid treatment all affect interpretation. Biotin in some hair or nail supplements can interfere with certain laboratory methods. Share supplement details and follow the laboratory's advice; any pause should match the product and assay, not a universal rule. [1] [2]
TSH comes from the pituitary, not the thyroid
TSH is a message sent from the pituitary gland to the thyroid. That explains why a high TSH can accompany an underactive thyroid: the body may be asking the thyroid to work harder. [3] [4]
Questions you might have
Why read TSH with free T4?
TSH is the pituitary signal; free T4 shows circulating unbound thyroid hormone. Their combination explains more than either result alone. [2]
Does TSH testing require fasting?
TSH alone usually does not have a fasting requirement. Other tests taken with it may; follow your appointment instructions. [1]
What comes next?
An unexpected or mildly changed TSH may need a repeat at an appropriate interval, often with free T4. Your clinician may consider thyroid antibodies if the cause is unclear. Avoid adjusting thyroid medicine or starting iodine supplements solely from one result. [1]
What does TSH mean alongside free T4, my symptoms, and any thyroid treatment?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
- MedlinePlus — Tsh Thyroid Stimulating Hormone Test
- American Thyroid Association — Thyroid Function Tests
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
- Moisio MA, Moisio EW. Understanding Laboratory and Diagnostic Tests. Delmar Publishers; 1998.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
