TSH receptor antibodies (TRAb) in Graves' disease stimulate the TSH receptor, mimicking TSH. This causes hyperthyroidism. The intracellular signaling cascade activated by TSH receptor stimulation and its downstream effects on thyroid hormone synthesis are:

• A TSH-R → Gi → reduced cAMP → reduced PKA → reduced thyroglobulin synthesis
• B TSH-R → Gs → increased cAMP → PKA activation → phosphorylation of transcription factors increasing thyroglobulin, NIS, TPO, and pendrin expression; Gq → IP3/DAG → PKC → iodine organification ✓
• C TSH-R → JAK/STAT pathway → direct thyroglobulin gene activation
• D TSH-R → receptor tyrosine kinase → MAP kinase → T3/T4 release without affecting synthesis

Explanation

The TSH receptor (TSHR) is a G-protein coupled receptor that activates two primary pathways. (1) Gs coupling → adenylyl cyclase → cAMP → PKA: this is the dominant pathway at physiological TSH concentrations, upregulating expression of sodium-iodide symporter (NIS) for iodide uptake, thyroglobulin synthesis, thyroid peroxidase (TPO) for iodide oxidation and tyrosine iodination, and thyroglobulin endocytosis/proteolysis for T3/T4 release. (2) Gq/11 coupling → phospholipase C → IP3 (Ca2+ release) + DAG (PKC): contributes to iodide organification (TPO activation) especially at supraphysiological TSH. TRAb in Graves' constitutively activates these pathways. JAK/STAT is used by cytokines; receptor tyrosine kinase by growth factors—neither is the TSHR mechanism.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.