Thyroid Blood Tests Explained: TSH, Free T3, Free T4

Why Thyroid Testing Matters

The thyroid gland, a butterfly-shaped structure at the base of your neck, regulates metabolism, body temperature, heart rate, and energy production. Dysfunction is surprisingly common—hypothyroidism affects roughly 5% of the US population, and subclinical thyroid disease is even more prevalent.[1] Because symptoms such as fatigue, weight changes, cold intolerance, and mood disturbances are nonspecific, blood tests are essential for an accurate diagnosis.

The Hypothalamic–Pituitary–Thyroid Axis

Understanding thyroid tests requires a brief look at the control system behind them. The hypothalamus releases thyrotropin-releasing hormone (TRH), which prompts the pituitary gland to secrete thyroid-stimulating hormone (TSH). TSH then signals the thyroid to produce thyroxine (T4) and triiodothyronine (T3). This negative-feedback loop means that when thyroid hormone levels fall, TSH rises to stimulate more production; when levels are excessive, TSH falls to pump the brakes.

This inverse relationship is why TSH is the most sensitive early indicator of thyroid dysfunction—it shifts before free T4 or free T3 move out of range.[1]

TSH: Thyroid-Stimulating Hormone

TSH is almost always the first thyroid test ordered. It is measured in milli-international units per liter (mIU/L). The standard reference range for adults is approximately 0.4–4.0 mIU/L, though some guidelines and laboratories use a slightly narrower range of 0.5–3.5 mIU/L.[3]

A high TSH (above 4.0 mIU/L) indicates the pituitary is working harder than normal to stimulate an underperforming thyroid—the hallmark of hypothyroidism. A low TSH (below 0.4 mIU/L) suggests the pituitary has dialled back stimulation because thyroid hormone levels are excessive, pointing toward hyperthyroidism or overtreatment with thyroid medication.[2] For a full reference-range breakdown, see our TSH biomarker page.

Subclinical hypothyroidism is defined as a TSH above the upper limit of normal with free T4 still within range.[4][5] Treatment decisions in this zone depend on TSH magnitude, symptoms, and individual risk factors such as pregnancy or cardiovascular disease.

Free T4: Thyroxine

T4 is the primary hormone secreted by the thyroid gland. Most circulating T4 is bound to carrier proteins such as thyroxine-binding globulin (TBG); only the unbound fraction—free T4 (FT4)—is biologically active. Normal free T4 in adults is typically 0.8–1.8 ng/dL (approximately 10–23 pmol/L), though ranges differ slightly between assays.[3]

Free T4 is measured alongside TSH when initial screening suggests abnormality, or when central (pituitary) thyroid disease is suspected. A low FT4 paired with a high TSH confirms primary hypothyroidism. A high FT4 with a suppressed TSH confirms overt hyperthyroidism. An FT4 below normal with a low or normal TSH raises the possibility of central hypothyroidism, where the problem lies in the pituitary rather than the thyroid. More details are available on our free T4 biomarker page.

Free T3: Triiodothyronine

T3 is the metabolically active form of thyroid hormone. Although the thyroid secretes some T3 directly, the majority is produced in peripheral tissues by the conversion of T4 to T3 via the enzyme deiodinase.[6] Free T3 (FT3) normal range is approximately 2.3–4.1 pg/mL (3.5–6.5 pmol/L).

FT3 is not routinely tested in all patients, but it becomes important in specific situations. In hyperthyroidism, FT3 can be elevated even when FT4 remains borderline—a pattern called T3 toxicosis.[2] FT3 is also monitored in patients on combination T4/T3 replacement therapy and in those with suspected conversion problems, such as the low-T3 syndrome seen in severe non-thyroidal illness. Visit our free T3 biomarker page for detailed interpretation guidance.

Interpreting the Panel Together

Thyroid tests should rarely be interpreted in isolation. The most common patterns are as follows. Primary hypothyroidism presents with high TSH and low FT4. Subclinical hypothyroidism shows high TSH with normal FT4 and FT3. Overt hyperthyroidism produces suppressed TSH with elevated FT4 and/or FT3. Subclinical hyperthyroidism is defined by suppressed TSH with normal FT4 and FT3. Secondary (central) hypothyroidism is an uncommon pattern of low TSH with low FT4, requiring pituitary evaluation.[1][3]

Thyroid antibodies—anti-TPO and anti-thyroglobulin—are not part of the basic panel but may be added when autoimmune thyroid disease (Hashimoto's thyroiditis or Graves' disease) is suspected.

Factors That Affect Thyroid Test Results

Several factors can shift thyroid values without representing true thyroid disease. Biotin supplementation at high doses (above 5 mg/day) interferes with immunoassays and can falsely lower TSH and falsely elevate or lower thyroid hormones. Acute illness suppresses TSH transiently. Pregnancy raises TBG, making total T4 unreliable—free T4 and TSH are preferred during gestation. Medications including amiodarone, lithium, and certain glucocorticoids also alter thyroid parameters.

When to See a Doctor

A TSH outside the reference range on a single test does not always warrant immediate treatment, as transient fluctuations occur. Confirming an abnormal result with repeat testing 4–8 weeks later is common practice. However, markedly abnormal values, symptoms of thyroid dysfunction, or results suggesting central hypothyroidism warrant prompt clinical evaluation. Tracking thyroid values over time using an app like Health3 helps you and your physician identify trends and respond appropriately.

Track Your Biomarkers With Health3

Scan your lab results, explore biomarker interactions, and track trends over time with the Health3 app.