Thyroid hormones (T3 and T4) act on nuclear receptors to increase transcription. Which of the following best explains the calorigenic effect (thermogenesis) of thyroid hormones?
- A Increased expression of Na+/K+-ATPase in most body tissues, consuming more ATP and generating more heat; also upregulation of mitochondria and uncoupling proteins ✓
- B Direct activation of brown adipose tissue UCP-1 independent of sympathetic stimulation
- C Direct stimulation of the hypothalamic thermogenic centre raising the set-point
- D Activation of glycolysis, bypassing oxidative phosphorylation and generating heat directly
Explanation
Thyroid hormones (primarily T3, acting on TRα and TRβ nuclear receptors) increase BMR and thermogenesis through several mechanisms: (1) transcriptional upregulation of Na+/K+-ATPase in virtually all cells — increased pump activity consumes ATP, increasing heat production; (2) increased mitochondrial biogenesis, increasing oxidative capacity; (3) upregulation of mitochondrial uncoupling proteins (UCP2, UCP3), allowing proton leak and heat generation without ATP synthesis. The overall metabolic rate increases 60–100% in hyperthyroidism. Thyroid hormones also enhance sensitivity to catecholamines, amplifying sympathetic calorigenic effects.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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