What TSH, Free T4 and Free T3 Actually Measure

The three thyroid tests most panels start with

A thyroid test order rarely means a single test. The three that appear most often — TSH, free T4 and free T3 — measure different points in the same conversation between your brain and your thyroid gland. Knowing which point each one measures is what makes a result sheet readable. The American Thyroid Association lists these among the core thyroid function tests.

Test	What it measures	Made by	Direction of the signal
TSH (thyroid-stimulating hormone)	How hard the pituitary is pushing the thyroid to produce hormone. It measures thyroid output indirectly.	Pituitary gland	High TSH usually means the pituitary is pushing harder because thyroid hormone levels are low. Low TSH usually means it is easing off.
Free T4	The unbound share of thyroxine, the main hormone the thyroid releases, circulating and available to tissues.	Thyroid gland	High free T4 points to more hormone available than the pituitary expects; low points to less.
Free T3	The unbound share of triiodothyronine, the form that binds receptors inside cells and does most of the metabolic work.	Mostly converted from T4 in the liver and other tissues, with a small amount released directly by the thyroid	Free T3 shows how much of the active form is actually available.

How TSH, free T4 and free T3 fit together

They form a feedback loop. The hypothalamus releases TRH, which signals the pituitary to release TSH. TSH signals the thyroid to take up iodine and build T4, plus a little T3. T4 travels through the blood and tissues convert it to T3. When enough thyroid hormone is circulating, the hypothalamus and pituitary cut back their output; when levels drop, they increase it.

Because of that loop, TSH is a responding signal rather than a direct measurement of the gland. It often moves before free T4 or free T3 fall outside their printed reference ranges, which is why it is frequently the first test ordered. The trade-off is that TSH can be affected by pituitary problems and by some medications, so a doctor may add free T4 and free T3 to see the whole picture.

Why "free" T4 and T3 rather than total

Most T4 and T3 in the blood is attached to carrier proteins, mainly thyroxine-binding globulin. Bound hormone is a reservoir; only the unbound fraction can enter cells. Total T4 and total T3 measure bound plus free together, so anything that changes carrier protein levels — pregnancy, estrogen-containing medications, liver or kidney conditions — can move the total number without changing how much hormone tissues actually see. Free T4 and free T3 avoid that distortion, which is why they are the versions ordered most often.

What else a thyroid panel may include

• Total T4 and total T3 — added when binding proteins are a specific concern.
• Thyroid peroxidase (TPO) and thyroglobulin antibodies — proteins the immune system makes against thyroid tissue. They signal autoimmune activity, not how much hormone is circulating.
• Thyroglobulin — used mainly in follow-up of thyroid cancer, not as a general function test.
• Reverse T3 — an inactive form, measured rarely and interpreted cautiously.

There is no single standardized "thyroid panel," so what gets bundled varies by lab and by who ordered it. MedlinePlus covers the broader range of thyroid conditions, and the Lab Tests Online resource from AACC catalogs individual tests and what each is used for.

How to order a thyroid test

1. Decide which tests you want. TSH alone, TSH plus free T4, and a fuller panel with free T3 and antibodies are three different orders at three different prices.
2. Confirm whether your state requires a physician order. Many direct-access lab services include clinician review of the order in the price; a few states require you to see your own provider first.
3. Tell the lab about supplements and medications. Biotin is a known interferent in some thyroid immunoassays, and a clinician can advise on timing.
4. Book a consistent draw time. TSH follows a daily rhythm and tends to run higher overnight, so testing at roughly the same hour makes a trend easier to read.
5. Fasting is usually not required for thyroid tests, though a lab may bundle other tests that do require it.

What thyroid testing costs

Self-pay prices are set by the lab and the ordering channel, not by how hard the test is to run. A few things drive the number:

• How the order is bundled. A stand-alone TSH is usually the least expensive order. Adding free T4, free T3 and antibodies adds line items, each with its own reagent and labor cost.
• The collection fee. Almost every quote includes a venipuncture fee, and some listings hide it until checkout. Compare the all-in figure.
• Whether clinician review is included. Direct-access services that supply a physician order build that cost into the price.
• Turnaround. Routine thyroid panels are typically reported within a few business days; expedited reporting costs more.

When you compare, match the test list first. A TSH-only listing and a panel with antibodies are different products, and the gap in price reflects the number of assays run rather than better or worse care.

Reading the numbers without over-reading them

Reference ranges come from the lab that ran your sample and describe the middle portion of results from a reference population. They are not a dividing line between well and unwell. TSH in particular shifts with age, pregnancy, acute illness and some medications. The MedlinePlus guide to understanding lab results explains why a value outside the printed range is a prompt for a conversation, not a verdict.

Free T4 and free T3 results also vary between assay platforms, so a number from one lab is not always directly comparable to a number from another. If you are tracking a trend, using the same lab each time removes one source of noise.

TestCosts.com is a price directory, not a laboratory or a medical provider. It does not collect samples, run tests or interpret results. Any thyroid test result should be discussed with a licensed clinician.