Diseases and conditions associated with thyroid hormone imbalance (both deficiency and excess), as this is clinically very important.
🔹 1️⃣ Thyroid hormone deficiency (hypothyroidism)
Occurs when thyroxine (T₄) and triiodothyronine (T₃) levels are below normal → reduced metabolic activity.
⚠️ Diseases and conditions:
✅ Primary hypothyroidism (most common):
- Hashimoto’s thyroiditis (autoimmune destruction)
- Iodine deficiency
- Post-thyroidectomy or radioiodine therapy
✅ Congenital hypothyroidism:
- Leads to cretinism: severe intellectual disability, growth retardation, deaf-mutism if untreated in infancy.
✅ Secondary hypothyroidism:
- Pituitary failure (↓ TSH) → ↓ thyroid hormone.
✅ Tertiary hypothyroidism:
- Hypothalamic disease (↓ TRH).
✅ Subclinical hypothyroidism:
- Normal thyroid hormones but elevated TSH.
🩺 Signs and symptoms of hypothyroidism:
- Fatigue, weight gain
- Cold intolerance
- Bradycardia, constipation
- Depression, dry skin, hair thinning
- Slow reflexes
- In children: growth retardation, delayed development
🔹 2️⃣ Thyroid hormone excess (hyperthyroidism / thyrotoxicosis)
Occurs when T₄ and/or T₃ levels are elevated → increased metabolic rate.
⚠️ Diseases and conditions:
✅ Graves’ disease (most common):
- Autoimmune stimulation of TSH receptors → ↑ thyroid hormone production.
✅ Toxic multinodular goiter:
- Autonomous nodules producing excessive thyroid hormone.
✅ Toxic adenoma:
- Single hyperfunctioning nodule.
✅ Subacute thyroiditis:
- Inflammation leading to transient release of stored thyroid hormones.
✅ Factitious thyrotoxicosis:
- Excess exogenous thyroid hormone intake.
✅ T₃ toxicosis:
- Early hyperthyroidism where T₃ is elevated but T₄ may still be normal.
🩺 Signs and symptoms of hyperthyroidism:
- Weight loss despite normal/increased appetite
- Heat intolerance, sweating
- Tachycardia, palpitations
- Tremor, anxiety, nervousness
- Diarrhea
- Hyperreflexia
- Muscle weakness
- Graves’ disease can include exophthalmos (eye bulging)
🔹 3️⃣ Other conditions related to altered thyroid hormones
✅ Sick euthyroid syndrome (non-thyroidal illness syndrome):
- ↓ T₃ (early), later ↓ T₄, normal/low TSH
- Occurs during severe illness; thyroid is functionally normal.
✅ Iodine deficiency goiter:
- Compensatory thyroid enlargement due to inadequate thyroid hormone synthesis.
✅ Resistance to thyroid hormone (RTH):
- Rare disorder where tissues are less responsive to thyroid hormones → high T₃/T₄ but variable symptoms.
🔹 Summary table:
| Imbalance | Diseases/Conditions | Key features |
| 🔻 Thyroid hormone deficiency | Hashimoto’s thyroiditis, iodine deficiency, congenital hypothyroidism (cretinism), post-thyroidectomy, secondary/tertiary hypothyroidism | Fatigue, cold intolerance, bradycardia, constipation, weight gain, slowed growth (children), cognitive impairment |
| 🔺 Thyroid hormone excess | Graves’ disease, toxic multinodular goiter, toxic adenoma, thyroiditis, T₃ thyrotoxicosis, factitious thyrotoxicosis | Weight loss, tachycardia, heat intolerance, tremor, anxiety, diarrhea, hyperactivity |
| ⚠️ Other conditions | Sick euthyroid syndrome, iodine deficiency goiter, thyroid hormone resistance | Variable; dependent on cause and compensation |
✅ Key concept:
- Thyroid hormone imbalance causes profound systemic effects due to their central role in metabolism, growth, cardiovascular regulation, and nervous system function.