TSH vs T4 vs T3: What Each Thyroid Test Measures, Which One to Start With, and How to Read Them Together

The thyroid system is a thermostat. The pituitary releases TSH (thyroid-stimulating hormone) to tell the thyroid how hard to work; the thyroid responds by releasing mostly T4 (thyroxine), a storage-and-transport hormone; and the body's tissues convert T4 into T3 (triiodothyronine), the form that actually acts on cells. The pituitary watches the result and adjusts TSH: when thyroid hormone falls, TSH rises to compensate, and when hormone rises, TSH falls. That feedback is why TSH is the most sensitive test — it moves logarithmically, so a small drop in T4 produces a large rise in TSH long before T4 itself leaves the normal range.

TSH is reported in mIU/L, with a typical adult reference range of roughly 0.4–4.5 (laboratories vary, and the upper limit is debated; guidelines treat values in the 4–10 range with a normal free T4 as subclinical hypothyroidism rather than disease). A high TSH means the pituitary is calling for more hormone — the thyroid is underperforming. A low TSH means it is telling the thyroid to slow down — there is too much hormone, whether from an overactive gland or from medication.

TSH is the guideline-recommended first test for screening and for monitoring levothyroxine therapy. Its limits: it is unreliable when the pituitary itself is the problem (rare), it lags real changes by several weeks (so it is checked 6–8 weeks after any dose change, not sooner), and it fluctuates with time of day, acute illness, recovery from illness and the first trimester of pregnancy. Biotin supplements can interfere with some TSH assays. The TSH reference page covers ranges, causes and the pregnancy-specific targets.

T4 is the hormone the thyroid mainly produces. More than 99% of it in blood is bound to carrier proteins — thyroxine-binding globulin, albumin and others — and only the unbound fraction, free T4, can enter cells. A total T4 test counts both bound and free, which means it rises and falls with the binding proteins: pregnancy, estrogen and oral contraceptives raise total T4 without changing thyroid function, while some illnesses and medications lower it. Free T4 sidesteps that, which is why guidelines and most laboratories now default to it. Reference ranges run about 0.8–1.8 ng/dL depending on the assay.

Free T4's job is to confirm and grade what TSH suggests. High TSH with low free T4 is overt hypothyroidism; high TSH with normal free T4 is subclinical. Low TSH with high free T4 is overt hyperthyroidism. It is also the test that stands in for TSH when the pituitary cannot be trusted — after pituitary surgery or disease, and in the first weeks after a treatment change when TSH has not yet caught up. Details on the free T4 reference page.

T3 is three to four times more potent than T4 and is the hormone that binds thyroid receptors in cells. About 80% of it is made outside the thyroid by converting T4, so in most people T3 tracks T4 and adds little to a diagnosis of hypothyroidism — it is often the last value to fall, and guidelines do not recommend T3 for diagnosing or monitoring an underactive thyroid. As with T4, free T3 (roughly 2.3–4.2 pg/mL) is preferred over total T3 for the same binding-protein reasons.

T3 earns its place in three situations. In hyperthyroidism, T3 can rise before or out of proportion to T4 (T3 toxicosis), so a suppressed TSH with a normal free T4 is followed by free T3. In people taking T3-containing therapy (liothyronine, desiccated thyroid), free T3 is part of monitoring, timed relative to the dose. And when symptoms persist despite a normal TSH and free T4, free T3 — and sometimes reverse T3 — is checked to look for conversion problems, though the evidence that treating those numbers helps is limited, and mainstream guidelines do not endorse dosing to a T3 target. The free T3 reference page has the ranges.

• Fatigue, weight change, feeling cold or hot, hair loss, and no thyroid history: TSH + free T4. TSH alone is enough for a pure screen, but the pair costs little more and saves a second draw if TSH is off.
• Known hypothyroidism on levothyroxine: TSH, drawn 6–8 weeks after any dose change and then annually, taken before your morning dose. Free T4 is added if TSH and symptoms disagree.
• Racing heart, weight loss, tremor, heat intolerance, anxiety — suspected hyperthyroidism: TSH, free T4 and free T3, plus thyroid antibodies (TRAb/TSI) to identify Graves' disease.
• Family history of thyroid disease or an autoimmune condition: TSH + free T4 with TPO antibodies, which predict who progresses to hypothyroidism. The Complete Thyroid Panel bundles TSH, free T4, free T3 and antibodies.
• Pregnant or planning pregnancy: TSH (with free T4), read against trimester-specific ranges — the first-trimester TSH range is lower than the general adult range.
• On desiccated thyroid or liothyronine: TSH and free T3 (with free T4), timed as your prescriber directs relative to the dose.

Practical rules: draw in the morning for consistency (TSH peaks overnight and is lowest in the afternoon); if you take levothyroxine, draw before the dose; stop biotin supplements for two to three days beforehand because they interfere with some assays; and do not test during an acute illness or within six weeks of recovering from one, when both TSH and T3 behave abnormally.

A single abnormal TSH is repeated — usually with free T4 and TPO antibodies — before anything is concluded, because transient shifts from illness, recovery and lab variation are common. Confirmed overt hypothyroidism is treated with levothyroxine and monitored by TSH; subclinical hypothyroidism is a judgment call that depends on how high TSH is, whether antibodies are present, symptoms, age and pregnancy plans, and is often simply watched. Hyperthyroidism needs an endocrinology evaluation for the cause before treatment, since Graves' disease, nodules and thyroiditis are handled differently. Normal TSH and free T4 with persistent symptoms means looking beyond the thyroid — iron, B12, vitamin D, sleep, mood and other hormones — rather than at T3 numbers alone.

All three tests are routine draws at Quest and Labcorp, orderable through TestWell with physician review; results post within a few business days with the laboratory's ranges.