Overview
Triiodothyronine (T3) is a crucial thyroid hormone essential for regulating the body’s metabolism, growth, development, heart rate, and nervous system activity. Although the thyroid gland produces only a small amount of T3, it is the most biologically active thyroid hormone, possessing 3–4 times greater potency than T4. Its chemical formula is C₁₅H₁₂I₃NO₄, and the hormone is built on a tyrosine amino acid backbone with three iodine atoms, giving it a molecular weight of 650.97 g/mol.
T3 is produced partly by the thyroid gland (20–25%) and mostly by conversion of T4 to T3 through peripheral tissues (75–80%). It has a half-life of 1 day and plays a vital role in energy production, heart function, nervous system stimulation, gastrointestinal motility, and brain development.
T3 travels in blood mostly bound to proteins, primarily Thyroxine Binding Globulin (TBG), which ensures stability and prevents rapid excretion. Only about 0.04% of T3 remains unbound, and this free portion is responsible for its biological activity.
The normal reference range for T3 is 60–200 ng/dL, though slight variations may occur between laboratories.
Symptoms
T3 imbalances—either high or low—produce distinct clinical symptoms.
Symptoms of High T3 (Hyperthyroidism / Thyrotoxicosis):
- Weight loss
- Rapid or irregular heartbeat
- Nervousness, irritability
- Shaky hands
- Trouble sleeping
- Sweating or heat intolerance
- Muscle weakness
- Frequent bowel movements
- Fatigue despite hyperactivity
- Goitre (thyroid enlargement)
Symptoms of Low T3 (Hypothyroidism):
- Dry skin
- Weight gain
- Puffy face
- Hoarse voice
- Coarse hair and skin
- Hair thinning
- Severe fatigue
- Bradycardia (slow heartbeat)
- Depression
- Memory problems
- Constipation
- Infertility or menstrual abnormalities
Symptoms in Children:
- Poor growth or short stature
- Delayed permanent teeth
- Delayed puberty
- Poor mental development
- Difficulty concentrating
- Poor school performance
- Eye bulging (in some cases)
Symptoms in Infants:
- Poor feeding
- Poor weight gain
- Irritability
- Constipation
- Dry skin
- Hoarse cry
- Enlarged tongue
- Jaundice
- Tachycardia
- Umbilical hernia
Causes
The PDF lists distinct causes for low T3 and high T3 levels.
Low T3 – Causes:
- Severe short-term or long-term illness
- Thyroiditis (e.g., Hashimoto’s disease)
- Starvation
- Underactive thyroid gland (Myxedema)
- Medications such as steroids and amiodarone
- Severe systemic illness
High T3 – Causes:
- Overactive thyroid gland (e.g., Graves’ disease)
- T3 thyrotoxicosis (rare)
- Thyroid nodules
- Thyroiditis
- Toxic nodular goitre
- Use of thyroid medications or supplements
- Liver disease
These causes affect T3 production, conversion, or release, leading to significant metabolic and systemic changes.
Risk Factors
Conditions that predispose individuals to abnormal T3 levels include:
Risk Factors for Low T3:
- Autoimmune thyroiditis
- Chronic illness or severe infections
- Starvation or poor nutrition
- Long-term use of medications impacting thyroid function
- Underactive thyroid or pituitary dysfunction
Risk Factors for High T3:
- Hyperthyroidism (e.g., Graves’ disease)
- Presence of thyroid nodules or multinodular goitre
- Excessive intake of thyroid hormone supplements
- Liver dysfunction affecting hormone metabolism
- Family history of thyroid disorders
Women, infants, and individuals with autoimmune tendencies have enhanced vulnerability to T3 abnormalities.
Prevention
Prevention focuses on early diagnosis, proper testing, and lifestyle considerations.
Testing & Sample Collection (from PDF):
- Patient must remain overnight fasting.
- Collect 3 mL blood in a plain red-capped tube.
- Separate serum as early as possible.
- Avoid lipemic (milky) serum.
- If lipemia occurs, use high-speed centrifugation (10,000 × g for 15 minutes) to clear the sample.
Clinical Prevention:
- Regular thyroid screening for individuals with family history or symptoms.
- Early evaluation of unexplained weight changes, fatigue, or menstrual irregularities.
- Monitoring T3 in patients on steroids, amiodarone, or thyroid supplements.
- Maintaining nutritional balance to prevent starvation-induced low T3.
Lifestyle Prevention:
- Adequate iodine intake through diet
- Good sleep and stress management
- Avoid self-medicating with thyroid supplements
- Regular medical checkups for infants and children to detect thyroid dysfunction early
