T4, or Thyroxine, is one of the two main thyroid hormones, the other being T3 (Triiodothyronine). It is produced in larger quantities by the thyroid gland but is less biologically active than T3.
T4 acts primarily as a precursor to T3 — most of its physiological effects occur after it is converted into T3 in target tissues like the liver, kidneys, and brain.
🧠 Where Is T4 Made?
T4 is made by the thyroid gland, located in the front of your neck. Its production is controlled by the hypothalamic–pituitary–thyroid (HPT) axis:
- The hypothalamus releases TRH (Thyrotropin-Releasing Hormone)
- TRH signals the anterior pituitary to release TSH (Thyroid-Stimulating Hormone)
- TSH stimulates the thyroid gland to produce T4 and T3
- About 90% is T4
- About 10% is T3
🧬 T4 vs. T3 – Key Differences
| Feature | T4 (Thyroxine) | T3 (Triiodothyronine) |
|---|---|---|
| Iodine atoms | 4 | 3 |
| Amount produced | More | Less |
| Activity level | Less active | More active |
| Half-life | Long (7–10 days) | Short (about 1 day) |
| Role | Storage form, converted to T3 | Active form that acts on tissues |
🔬 What Does T4 Do?
Although T4 itself has some biological activity, its main function is to serve as a reservoir for the production of T3, the active thyroid hormone. Once converted, T3 influences:
- 🔥 Metabolic rate
- 🧠 Brain development
- 🌡️ Body temperature regulation
- ❤️ Heart rate and cardiovascular function
- 💪 Muscle and bone growth
- 🧃 Energy production and digestion
🧪 Forms of T4 in the Blood:
- Total T4 – Measures both bound (attached to proteins) and free (active) T4.
- Free T4 (FT4) – Measures only the unbound, active hormone available to enter tissues.
Doctors typically rely on Free T4 to assess thyroid function.
📉 Low T4 May Indicate:
- Hypothyroidism (underactive thyroid)
- Hashimoto’s thyroiditis (autoimmune thyroid disease)
- Iodine deficiency
- Pituitary or hypothalamic disorders (secondary or tertiary hypothyroidism)
Symptoms of Low T4:
- Fatigue
- Cold intolerance
- Weight gain
- Depression
- Dry skin
- Constipation
- Slow heart rate
📈 High T4 May Indicate:
- Hyperthyroidism (overactive thyroid)
- Graves’ disease (autoimmune)
- Thyroid nodules or tumors
- Overmedication with thyroid hormone
Symptoms of High T4:
- Weight loss
- Rapid heartbeat
- Anxiety
- Heat intolerance
- Sweating
- Tremors
- Diarrhea
💊 T4 Replacement Therapy
Synthetic T4 (called levothyroxine) is the standard treatment for hypothyroidism. It restores hormone levels and allows the body to naturally convert T4 to T3 as needed.
Brand names include:
- Synthroid
- Levoxyl
- Euthyrox
- Eltroxin
🧠 Summary Table:
| Feature | T4 (Thyroxine) |
|---|---|
| Produced by | Thyroid gland |
| Stimulated by | TSH from the anterior pituitary |
| Main role | Precursor to T3; regulates metabolism and energy |
| Active form | Free T4 (FT4) |
| Converted to | T3 in liver, kidneys, and other tissues |
| Measured in tests | Total T4 and Free T4 |
| Clinical relevance | Diagnosing and managing thyroid disorders |