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Graves Disease II: Pathophysiology
Graves’ disease is an autoimmune disorder characterized by the production of thyroid-stimulating immunoglobulins (TSI) that activate TSH receptors, le…
Graves' disease begins when the immune system fails to maintain tolerance, allowing plasma cells to produce and release thyroid-stimulating immunoglobulins or TSI.
These antibodies bind to thyroid-stimulating hormone receptors or TSH-R on thyroid follicular cells and mimic the action of thyroid-stimulating hormone or TSH.
With the receptors continuously overstimulated by the antibodies, the thyroid gland produces and releases large amounts of thyroxine or T4 and triiodothyronine or T3 without normal regulatory control.
Microscopically, follicular cells become tall and crowded, vascularity increases, and colloid stores are rapidly depleted, leaving wavy edges.
The same immune response targets fibroblasts expressing TSH receptors in the orbit and skin.
It causes swelling around the eyes, known as periorbital edema, protrusion of the eyes called exophthalmos, and thickened patches on the shins, referred to as pretibial myxedema.
Excess T4 and T3 in the blood raise the metabolic rate, increasing heat production and heart rate.
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Q1: How do thyroid-stimulating immunoglobulins cause Graves' disease?
In Graves' disease, thyroid-stimulating immunoglobulins (TSI) are autoantibodies produced when immune tolerance fails. These antibodies bind to TSH receptors on thyroid follicular cells and mimic thyroid-stimulating hormone, continuously overstimulating the receptors. This bypasses normal feedback control, causing excessive production and release of T4 and T3 without regulation.
Q2: What microscopic changes occur in thyroid follicular cells during Graves' disease?
Thyroid follicular cells become tall and columnar, increase in number, and crowd together. Vascularity increases significantly throughout the gland. Colloid stores are rapidly depleted due to excessive hormone utilization, leaving characteristic wavy or scalloped edges on the follicles.
Q3: Why does Graves' disease cause eye protrusion and orbital swelling?
The same autoimmune response targeting thyroid cells also affects fibroblasts expressing TSH receptors in the orbit. Glycosaminoglycan deposition in orbital fibroblasts causes edema and tissue swelling, leading to periorbital edema and exophthalmos, or eye protrusion. This extrathyroidal manifestation occurs independently of thyroid hormone levels.
Q4: What is pretibial myxedema and how does it develop in Graves' disease?
Pretibial myxedema is thickened, swollen skin patches appearing on the shins. It develops when the autoimmune response targets TSH receptors on skin fibroblasts, particularly in the pretibial region. Glycosaminoglycan accumulation in these fibroblasts causes localized edema and tissue thickening characteristic of this condition.
Q5: How do elevated thyroid hormones affect metabolic rate and heart function in Graves' disease?
Excess T4 and T3 in the blood increase basal metabolic rate and heat production. Elevated thyroid hormones also increase sensitivity to catecholamines, raising heart rate and cardiac output. This hypermetabolic state produces tachycardia, palpitations, heat intolerance, and increased sweating as patients struggle to dissipate excess metabolic heat.
Q6: Why does negative feedback control fail to suppress thyroid hormone production in Graves' disease?
Although elevated T3 and T4 normally suppress pituitary TSH through negative feedback, this regulatory mechanism fails in Graves' disease. Antibody-mediated stimulation of TSH receptors persists independently of TSH levels, continuously driving thyroid hormone synthesis. The immune attack bypasses the normal hypothalamic-pituitary-thyroid axis control.
Q7: What systemic symptoms result from the hypermetabolic state in Graves' disease?
The hypermetabolic state produces weight loss despite increased appetite, fine tremors, anxiety, and hyperactivity. Patients experience tachycardia, palpitations, muscle weakness, and heat intolerance with excessive sweating. In severe cases, arrhythmias or thyroid storm may develop, representing life-threatening complications of uncontrolled hyperthyroidism.