Home Biomarkers T3 (Triiodothyronine)

T3 (Triiodothyronine)

Thyroid Biomarker

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Collection Type: Blood

Body System

Related System: Thyroid

Overview

Triiodothyronine (T3) is one of the two primary thyroid hormones and is the more biologically active form that regulates metabolism, heart rate, temperature, and many cellular processes. The blood test measures either total T3 (T3 bound to transport proteins plus free T3) or free T3 (unbound, active hormone). It is used to help evaluate suspected thyroid dysfunction especially hyperthyroidism and to monitor treatment. Symptoms prompting testing include unexplained weight change, palpitations, tremor, heat or cold intolerance, fatigue, hair loss, menstrual changes, and mood alterations. Results vary with age (newborns higher; older adults often lower), pregnancy (total T3 may rise due to increased binding), sex hormones, and certain medications.

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Test Preparation

No special preparation is required

Why Do I Need This Test

Profile: part of a thyroid function panel (often alongside TSH and T4) - Symptoms: weight loss/gain, palpitations, fatigue, temperature intolerance, hair loss, mood or menstrual changes - Diagnoses/monitoring: hyperthyroidism, “T3-toxicosis,” hypothyroidism assessment and treatment monitoring - Reasons for abnormal levels: overproduction (Graves’, toxic nodules), underproduction, altered peripheral conversion, non-thyroidal illness, medications - Biological meaning: high T3 ⇒ excess thyroid activity; low T3 ⇒ reduced thyroid activity or impaired conversion - Lifestyle/factors: fasting, severe illness, certain drugs (amiodarone, steroids, lithium), supplements (biotin) can alter results - Family history of thyroid disease increases need for testing

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Understanding Test Results

Values above the stated upper limits (Total T3 >200 ng/dL or Free T3 >4.2 pg/mL) indicate excess thyroid hormone activity and commonly suggest hyperthyroidism from causes such as Graves’ disease, toxic multinodular goiter, a T3-secreting adenoma, thyroiditis (transient release), or exogenous T3 ingestion.
Moderately elevated T3 with normal T4 and suppressed TSH can indicate T3-toxicosis.
Values below the lower limits (Total T3 <80 ng/dL or Free T3 <2.3 pg/mL) suggest hypothyroidism, impaired peripheral conversion of T4→T3 (seen in non-thyroidal illness/euthyroid sick syndrome), pituitary/hypothalamic disease (when TSH is low), malnutrition, or effects of certain drugs.
Mild deviations warrant correlation with TSH and free T4; marked abnormalities require prompt endocrine assessment and treatment adjustments.

Normal Range

Total T3: 80-200 ng/dL OR 1.23-3.08 nmol/L; Free T3: 2.3-4.2 pg/mL OR 3.5-6.5 pmol/L

FAQs

Q: What does triiodothyronine T3 do?

A: Triiodothyronine (T3) is the active thyroid hormone that regulates metabolism and energy use. It increases basal metabolic rate, oxygen consumption and heat production; stimulates carbohydrate, fat and protein metabolism; raises heart rate and contractility; and supports normal growth, brain development and nervous-system function. T3 also influences cholesterol levels, thermoregulation and the expression of metabolic enzymes.

Q: What happens if T3 is low?

A: Low T3 (triiodothyronine) causes slowing of body functions: fatigue, weight gain, cold intolerance, dry skin, hair loss, constipation, slowed heart rate, muscle weakness, depression, cognitive slowing and heavier menstrual bleeding. Metabolism and cholesterol worsen, raising cardiovascular risk. In severe cases it can lead to myxedema and impaired organ function. Evaluation and thyroid hormone replacement can correct these effects.

Q: What happens if T3 is high?

A: High T3 (triiodothyronine) speeds up metabolism, causing weight loss, rapid or irregular heartbeat, palpitations, tremors, anxiety, heat intolerance, sweating, increased appetite, insomnia and muscle weakness. It can lead to menstrual changes, diarrhea, bone loss and higher risk of atrial fibrillation or hypertension. Causes include Graves’ disease, toxic nodules, thyroiditis or excess thyroid medication; severe cases may trigger a life‑threatening thyroid storm.

Q: What are the side effects of T3 triiodothyronine?

A: T3 (triiodothyronine) side effects include symptoms of excess thyroid: palpitations, rapid heartbeat, arrhythmias, chest pain, high blood pressure, anxiety, tremor, insomnia, nervousness, weight loss, heat intolerance and sweating, increased appetite, diarrhea, and muscle weakness. Long-term or high‑dose use can cause bone loss (osteoporosis), menstrual irregularities and increased cardiovascular risk; severe cases may trigger atrial fibrillation or heart failure.

Q: Can T3 cause weight loss?

A: Yes excess T3 (from hyperthyroidism or taking T3 unnecessarily) raises basal metabolic rate and often causes weight loss. However this loss can include muscle and bone, may be temporary, and carries dangers (arrhythmias, osteoporosis, heat intolerance). T3 should only be used when medically indicated; it is not a safe or recommended weight‑loss treatment.

Q: What organs does T3 affect?

A: Triiodothyronine (T3) acts on nearly every organ system: heart (rate and contractility), brain and peripheral nervous system (development, mood, cognition), liver and kidneys (metabolism, gluconeogenesis, cholesterol), skeletal muscle and bone (growth, turnover), adipose tissue (lipolysis and thermogenesis), gastrointestinal tract (motility), skin, and reproductive organs (fertility and menstrual function) influencing overall basal metabolic rate.