Home Biomarkers T4 (Thyroxine)

T4 (Thyroxine)

Thyroid Biomarker

Sample Needed

Collection Type: Blood

Body System

Related System: Thyroid

Overview

T4 (thyroxine) is a primary hormone produced by the thyroid gland; most circulating T4 is protein-bound and serves as a reservoir that is converted peripherally to the more active T3. A T4 blood test measures the amount of thyroxine (usually total T4) in the bloodstream and helps assess thyroid function. It is used when symptoms like unexplained weight change, fatigue, palpitations, heat/cold intolerance, hair thinning, constipation or menstrual irregularities appear. Abnormal T4 may indicate hyperthyroidism, hypothyroidism, thyroiditis, pituitary disease or effects of drugs and pregnancy. Levels vary with age (neonates have higher values), pregnancy (total T4 rises due to increased binding), sex, and factors affecting thyroid‑binding proteins.

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Test Preparation

Do not stop taking your thyroid medications on the day of the test unless otherwise advised by the doctor

Why Do I Need This Test

Profile: Thyroid profile (often alongside TSH and sometimes free T4/T3) - Symptoms: weight change, fatigue, mood changes, palpitations, thermoregulatory symptoms, hair/skin changes - Diagnoses/monitoring: hyperthyroidism, hypothyroidism, thyroiditis, response to therapy, or follow-up of pituitary disease - Causes of abnormal levels: primary thyroid disease, pituitary dysfunction, altered thyroid‑binding proteins (pregnancy, estrogen), medications, excess exogenous thyroid hormone - Biological meaning: high T4 indicates excess thyroid hormone or increased binding; low T4 indicates deficient thyroid hormone or reduced binding - Behaviors/factors: iodine intake, thyroid hormone supplementation, certain drugs, biotin supplements can skew results - Family history: autoimmune thyroid disease (Graves’, Hashimoto’s) increases need for testing

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Understanding Test Results

Values above 12.0 µg/dL suggest hyperthyroxinemia and may indicate hyperthyroidism (e.g., Graves’ disease), thyroid hormone overdose, or increased thyroid‑binding globulin (pregnancy, estrogen therapy).
Marked elevations (for example >20 µg/dL) are more likely with significant thyrotoxicosis and warrant prompt evaluation.
Values between 12.0 and the upper moderately elevated range may be seen in early/subclinical states or binding protein changes.
Values below 5.0 µg/dL indicate hypothyroxinemia and can reflect primary hypothyroidism, central (pituitary) hypothyroidism, or low binding proteins (nephrotic syndrome, severe illness).
Very low values (for example <2.0 µg/dL) suggest severe hypothyroidism and need urgent assessment.
Always interpret T4 together with TSH (and free T4) because alterations in binding proteins, pregnancy, medications, or assay interference (e.g., biotin) can change total T4 independently of tissue thyroid status.

Normal Range

5.0-12.0 µg/dL OR 64-154 nmol/L

FAQs

Q: What is the T4 in thyroxine?

A: T4, or thyroxine, is the thyroid gland’s main hormone containing four iodine atoms (tetraiodothyronine). It’s produced by follicular cells, circulates mostly bound to carrier proteins, and is converted in tissues to the more active T3. T4 helps regulate basal metabolic rate, heart rate, growth and development. Clinically, free T4 (unbound) is measured to assess thyroid function.

Q: What if thyroxine T4 is high?

A: If thyroxine (T4) is high, it suggests an overactive thyroid or excess hormone replacement. Symptoms include weight loss, palpitations, heat intolerance, tremor, anxiety, increased appetite and sweating. Causes: Graves’ disease, toxic nodules, thyroiditis or too much levothyroxine. Risks include atrial fibrillation and bone loss. Management: adjust medication, beta‑blockers for symptoms, repeat tests and endocrine review; seek urgent care for chest pain, severe breathlessness or high fever.

Q: What is a normal T4 level?

A: Normal T4 levels depend on the test. Free T4 is typically about 0.8–1.8 ng/dL (10–23 pmol/L). Total T4 usually ranges roughly 4.5–12.5 µg/dL (58–161 nmol/L). Reference ranges can vary between labs; clinicians interpret results alongside symptoms and other thyroid tests such as TSH and free T3. Always discuss abnormal values with your healthcare provider.

Q: Is it better to have high or low free T4?

A: Neither high nor low free T4 is better values within the laboratory reference range are optimal. High free T4 suggests hyperthyroidism, causing palpitations, weight loss and bone/heart risks. Low free T4 indicates hypothyroidism, causing fatigue, weight gain and cold intolerance. Management aims to normalize free T4; results must be interpreted alongside TSH and symptoms, so see a clinician if abnormal.

Q: What if T4 is low?

A: Low T4 suggests underactive thyroid (hypothyroidism) or pituitary dysfunction. Symptoms include fatigue, weight gain, cold sensitivity, constipation and slow pulse. Clinicians check TSH, free T4/T3 and sometimes antibodies to determine cause (e.g., autoimmune thyroiditis). Treatment is usually levothyroxine replacement with dose adjustments guided by TSH and symptoms; regular monitoring is important, especially in pregnancy and heart disease.

Q: When should I be worried about my T4 level?

A: Be concerned if your free T4 is outside the lab reference range or you have clear symptoms of hyperthyroidism (weight loss, palpitations, heat intolerance, tremor, anxiety) or hypothyroidism (fatigue, weight gain, cold intolerance, constipation, depression, hair loss). Seek urgent care for thyroid storm (high fever, very fast heartbeat, confusion) or myxedema coma (severe drowsiness, slowed breathing). Contact your clinician for abnormal results or pregnancy.