TSH, or Thyroid-Stimulating Hormone, is a hormone produced by the anterior pituitary gland (adenohypophysis) in the brain. Its primary function is to stimulate the thyroid gland to produce and release thyroid hormones:
- T3 (triiodothyronine)
- T4 (thyroxine)
These thyroid hormones control metabolism, energy production, body temperature regulation, and growth and development.
🧠 How TSH Works (Hormonal Feedback Loop)
The Hypothalamic–Pituitary–Thyroid (HPT) Axis:
- The hypothalamus releases TRH (thyrotropin-releasing hormone)
- TRH stimulates the anterior pituitary to release TSH
- TSH travels via the bloodstream to the thyroid gland
- The thyroid gland responds by releasing T3 and T4
- When T3/T4 levels rise, they inhibit further TRH and TSH release (negative feedback loop)
💊 What Does TSH Do?
TSH binds to receptors on the thyroid gland, triggering:
- Thyroid cell growth
- Uptake of iodine (essential for T3/T4 synthesis)
- Production and release of T3 and T4 hormones
Even though TSH doesn’t directly regulate metabolism, it controls the thyroid, which does regulate metabolism.
🧪 Why Is TSH Measured?
TSH is one of the most important blood tests used to evaluate thyroid function. It’s often tested when symptoms suggest:
- Hypothyroidism (underactive thyroid)
- Hyperthyroidism (overactive thyroid)
It is also monitored in:
- Thyroid hormone replacement therapy
- Thyroid nodule or goiter evaluation
- Pregnancy (thyroid disorders affect fetal development)
📉 Low TSH May Indicate:
- Hyperthyroidism (too much thyroid hormone)
- Pituitary or hypothalamic dysfunction
- Excess thyroid hormone replacement (overmedication)
📈 High TSH May Indicate:
- Hypothyroidism (thyroid is underactive and not producing enough hormones)
- Thyroid hormone resistance
- Thyroidectomy or damaged thyroid tissue
🧪 Normal TSH Range (Adults, varies slightly by lab):
- 0.4 to 4.0 mIU/L
(Some endocrinologists consider 0.5–2.5 optimal for general health)
TSH Levels and Thyroid Conditions:
| TSH Level | T3/T4 Levels | Likely Diagnosis |
|---|---|---|
| High TSH | Low T3/T4 | Primary Hypothyroidism |
| Low TSH | High T3/T4 | Hyperthyroidism |
| Low TSH | Low T3/T4 | Secondary Hypothyroidism (pituitary problem) |
🌿 Symptoms of Thyroid Imbalance:
🔻 If TSH is High (Hypothyroidism):
- Fatigue
- Cold intolerance
- Weight gain
- Depression
- Constipation
- Slow heart rate
🔺 If TSH is Low (Hyperthyroidism):
- Anxiety or irritability
- Heat intolerance
- Weight loss
- Rapid heart rate
- Tremors
- Insomnia
🧠 Summary:
| Feature | Description |
|---|---|
| Full Name | Thyroid-Stimulating Hormone |
| Produced by | Anterior pituitary gland (adenohypophysis) |
| Stimulated by | TRH from the hypothalamus |
| Acts on | Thyroid gland |
| Main effect | Stimulates production of T3 and T4 thyroid hormones |
| Measured for | Diagnosing thyroid disorders, guiding hormone therapy |