Diseases and conditions associated with triiodothyronine (T₃) imbalance, both deficiency and excess:
🔹 1️⃣ T₃ deficiency (low T₃ conditions)
Low T₃ is usually part of hypothyroidism, but in some cases, isolated low T₃ can occur (e.g., in illness).
⚠️ Diseases and conditions:
✅ Primary hypothyroidism (low T₄ and T₃)
- Hashimoto’s thyroiditis (autoimmune destruction)
- Iodine deficiency
- Post-thyroidectomy or radioiodine ablation
✅ Secondary/tertiary hypothyroidism
- Pituitary (↓ TSH) or hypothalamic (↓ TRH) disease → ↓ T₄ → ↓ T₃
✅ Sick euthyroid syndrome (Low T₃ syndrome)
- Critical illness: ↓ peripheral conversion of T₄ to T₃ despite normal thyroid gland
- Features: ↓ T₃, normal/low T₄, normal/low TSH
- Thought to be an adaptive response to reduce metabolism during stress
✅ Congenital hypothyroidism (cretinism)
- T₃ essential for neurodevelopment: untreated deficiency → irreversible intellectual disability, short stature
🩺 Signs and symptoms of T₃ deficiency:
- Fatigue, weight gain, cold intolerance
- Bradycardia, dry skin, constipation
- Slow reflexes, depression
- In children: growth retardation, delayed skeletal maturation, intellectual disability
🔹 2️⃣ T₃ excess (high T₃ conditions)
T₃ excess occurs in hyperthyroidism (thyrotoxicosis); sometimes disproportionate T₃ elevation (“T₃ thyrotoxicosis”) is seen.
⚠️ Diseases and conditions:
✅ Graves’ disease
- Most common cause of hyperthyroidism
- Autoimmune stimulation of TSH receptors → ↑ T₄ and T₃
✅ Toxic multinodular goiter / toxic adenoma
- Autonomous nodules producing excessive thyroid hormone (often ↑ T₃ predominance)
✅ Early thyroiditis (“leak” phase)
- Inflammation → release of stored T₃/T₄
✅ Factitious thyrotoxicosis
- Excessive exogenous thyroid hormone intake
✅ T₃ thyrotoxicosis
- Subtype of hyperthyroidism where T₃ is disproportionately elevated while T₄ may still be normal
- Early manifestation of toxic multinodular goiter or Graves’ disease
🩺 Signs and symptoms of T₃ excess:
- Weight loss despite increased appetite
- Heat intolerance, sweating
- Tachycardia, palpitations
- Tremor, anxiety, nervousness
- Diarrhea, hyperreflexia
- Muscle weakness
🔹 3️⃣ Special conditions related to altered T₃
✅ Sick euthyroid syndrome (“non-thyroidal illness syndrome”):
- ↓ T₃ (early), later ↓ T₄, ↓/normal TSH in severe illness.
- NOT true hypothyroidism; typically no replacement therapy needed.
✅ Resistance to thyroid hormone (RTH):
- Rare condition; elevated T₃ and T₄ but reduced tissue responsiveness
- Variable symptoms (may mimic hypo- or hyperthyroidism)
🔹 Summary table:
| Imbalance | Diseases / conditions | Key features |
| 🔻 T₃ deficiency | Primary hypothyroidism, secondary hypothyroidism, tertiary hypothyroidism, sick euthyroid syndrome, congenital hypothyroidism (cretinism) | Fatigue, bradycardia, cold intolerance, constipation, developmental delay (infants) |
| 🔺 T₃ excess | Graves’ disease, toxic multinodular goiter, toxic adenoma, early thyroiditis, factitious thyrotoxicosis, T₃ thyrotoxicosis | Weight loss, heat intolerance, tachycardia, tremor, anxiety, diarrhea |
✅ Key point:
Since T₃ is the active thyroid hormone, its imbalance has profound systemic effects:
- Deficiency = global slowing of metabolism
- Excess = hypermetabolic state