T3, or Triiodothyronine, is one of the two main thyroid hormones, the other being T4 (Thyroxine). It is the more active of the two and plays a major role in regulating your metabolism, energy levels, heart function, body temperature, and development.
🧠 Where Is T3 Made?
- The thyroid gland produces a small amount of T3 directly.
- However, most T3 (about 80–90%) is made by converting T4 (a less active form of thyroid hormone) into T3 in tissues such as the liver, kidneys, and brain.
- This conversion is done by enzymes called deiodinases.
🧬 T3 vs. T4 – Key Differences
| Feature | T3 (Triiodothyronine) | T4 (Thyroxine) |
|---|---|---|
| Atoms | 3 iodine atoms | 4 iodine atoms |
| Amount produced | Smaller quantity | Larger quantity |
| Activity level | More biologically active | Less active |
| Half-life | Short (about 1 day) | Long (about 7 days) |
| Main role | Immediate metabolic effects | Serves as a storage and precursor |
🔬 What Does T3 Do?
T3 influences the function of almost every cell in the body. It binds to receptors in the nucleus of cells and regulates gene expression to control metabolic processes.
Main Functions:
- 🔥 Increases metabolic rate (how fast cells convert nutrients to energy)
- ❤️ Regulates heart rate and strength
- 🧠 Supports brain development and mood regulation
- 🌡️ Maintains body temperature
- 🦴 Supports bone growth
- 🧃 Regulates digestion and glucose metabolism
- 💪 Maintains muscle strength and repair
🩺 When T3 Is Measured (Free T3 and Total T3)
- Free T3 (FT3): The active form of T3 that is not bound to proteins and available to tissues.
- Total T3: Includes both free and protein-bound T3 in the blood.
Doctors measure T3 to:
- Investigate hyperthyroidism (overactive thyroid)
- Help clarify thyroid function when TSH and T4 are inconclusive
- Monitor thyroid hormone therapy
📈 High T3 (T3 Toxicity or Hyperthyroidism)
Symptoms may include:
- Rapid heart rate (tachycardia)
- Anxiety, restlessness
- Weight loss despite normal eating
- Heat intolerance and sweating
- Tremors
- Diarrhea or frequent bowel movements
- Insomnia
Possible Causes:
- Graves’ disease (autoimmune hyperthyroidism)
- Toxic nodular goiter
- Overmedication with thyroid hormone (especially liothyronine)
📉 Low T3 (Hypothyroidism or Low T3 Syndrome)
Symptoms may include:
- Fatigue
- Weight gain
- Cold intolerance
- Depression or sluggish thinking
- Dry skin and hair loss
- Constipation
- Slowed heart rate
Possible Causes:
- Primary hypothyroidism (thyroid isn’t producing enough hormones)
- Chronic illness or starvation (known as “low T3 syndrome” or “euthyroid sick syndrome”)
- Pituitary dysfunction (if not enough TSH is produced)
🧠 Summary Table:
| Feature | T3 (Triiodothyronine) |
|---|---|
| Produced by | Thyroid gland and peripheral conversion from T4 |
| Biological activity | High (most active thyroid hormone) |
| Regulated by | TSH from the pituitary (via TRH from hypothalamus) |
| Functions | Controls metabolism, heart rate, body temp, and more |
| Measured as | Free T3 or Total T3 |
| Low levels | Seen in hypothyroidism or chronic illness |
| High levels | Seen in hyperthyroidism or overmedication |