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thyroid wt cause

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Here's a concise breakdown of how the thyroid gland affects body weight and the causes:

Thyroid & Body Weight

Hyperthyroidism → Weight Loss

Excess thyroid hormone increases the basal metabolic rate (BMR) by up to 60–100% above normal, dramatically accelerating metabolism. This leads to:
  • Weight loss (despite increased appetite)
  • Diarrhea, heat intolerance, sweating, nervousness, rapid heart rate
  • Note: The weight loss is not always guaranteed because thyroid hormones also stimulate appetite, which can partially counterbalance the metabolic increase
Common causes of hyperthyroidism (and thus weight loss):
  1. Graves' disease (~85% of cases) — autoimmune; TSH-receptor antibodies stimulate the thyroid continuously
  2. Hyperfunctioning ("toxic") nodules — autonomously secreting nodule(s)
  3. Thyroiditis — transient hormone leak from inflamed gland

Hypothyroidism → Weight Gain

Severely reduced thyroid hormone drops BMR to about half of normal, causing:
  • Weight gain
  • Fatigue, cold intolerance, constipation, slowed movements, lethargy, edema
Common causes of hypothyroidism (and thus weight gain):
  1. Hashimoto's thyroiditis — autoimmune destruction of the gland
  2. Iodine deficiency
  3. Post-surgical / post-radioiodine ablation
  4. Secondary hypothyroidism — pituitary TSH deficiency

Key Physiological Mechanism

Thyroid hormones (T3/T4) increase enzymatic activity in nearly all body cells. When levels are high, more ATP is consumed and heat is generated, burning more calories. When levels are low, this cellular "engine" slows down, reducing caloric expenditure and causing fat and fluid accumulation.
— Guyton and Hall Textbook of Medical Physiology; Costanzo Physiology 7th Ed.; Robbins & Kumar Basic Pathology
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