Thyrotoxicosis - Pathophysiology, Pathogenesis and Morphology

(Model Answer - 10 Marks, MD Pathology)

1. Definition

2. Etiology (Causes)

3. Pathogenesis

A. General mechanism of thyrotoxic state

B. Pathogenesis of Graves disease (the prototype and commonest cause - focus of most exam answers)

1. Loss of self-tolerance to thyroid antigens, particularly the TSH receptor (TSHR), in a genetically susceptible individual (associated HLA alleles and CTLA4 polymorphisms; increased concordance in monozygotic twins).
2. B cells produce thyroid-stimulating immunoglobulin (TSI) - an IgG autoantibody against the TSHR that binds and activates the receptor independent of TSH, driving continuous cAMP-mediated stimulation of follicular cells → increased synthesis and release of T3/T4 and gland hypertrophy/hyperplasia.
3. Other TSHR antibodies may block TSH binding (TSH-binding inhibitory immunoglobulins) - coexistence of stimulating and blocking antibodies in the same patient can explain intermittent swings toward hypothyroidism.
4. Ophthalmopathy: TSH receptor is also expressed on orbital fibroblasts and adipocytes. Activated CD4+ T cells secrete cytokines that stimulate fibroblasts to produce excess glycosaminoglycans/extracellular matrix, which accumulate in the retro-orbital space along with lymphocytic infiltration, producing proptosis and extraocular muscle edema/fibrosis.
5. Dermopathy (pretibial myxedema): similar glycosaminoglycan deposition and lymphocytic infiltration in the dermis.

C. Pathogenesis of other causes (brief, for completeness)

• Toxic multinodular goiter: autonomous, TSH-independent nodules (often with activating somatic mutations in the TSHR or Gsα gene) that secrete hormone independent of pituitary control.
• Toxic adenoma: monoclonal proliferation of follicular cells, frequently harboring activating point mutations in the TSHR gene, causing constitutive receptor activation.
• Subacute/painless thyroiditis: follicular disruption (viral or autoimmune) causes leakage of stored colloid/hormone - a transient thyrotoxic phase without true glandular hyperfunction.

4. Morphology

Gross

• Thyroid is symmetrically, diffusely enlarged (in Graves disease) due to hypertrophy and hyperplasia of follicular cells.
• Gland is smooth, soft, and the capsule is intact; cut surface is fleshy and resembles muscle ("meaty").
• (Contrast: toxic multinodular goiter shows an irregular, multinodular gland; toxic adenoma shows a single encapsulated nodule.)

Microscopy (Graves disease - MORPHOLOGY section, Robbins p. 2018-2022)

• Follicular epithelial cells are tall, columnar, and crowded, often forming small papillary infoldings projecting into the follicular lumen - these papillae lack fibrovascular cores, distinguishing them from true papillary carcinoma.
• Colloid is scant and pale, with scalloped ("moth-eaten") margins due to active resorption at the apical cell border.
• Lymphoid infiltrate in the interstitium, predominantly T cells with fewer B cells/plasma cells; scattered germinal centers may be present.
• Extrathyroidal changes: generalized lymphoid hyperplasia; orbital tissue shows edema (mucopolysaccharide/glycosaminoglycan deposition), lymphocytic infiltration (mostly T cells), and later fibrosis of extraocular muscles; dermis in pretibial myxedema shows glycosaminoglycan deposition with lymphocytic infiltrate.

5. Clinical correlation (brief, high-yield for exam)

• Constitutional: weight loss despite increased appetite, heat intolerance, warm moist skin.
• CVS: tachycardia, palpitations, atrial fibrillation, high-output cardiac failure in the elderly.
• GI: hypermotility, diarrhea.
• Neuromuscular: tremor, anxiety, proximal myopathy.
• Ocular: lid lag, wide staring gaze (sympathetic overactivity); true infiltrative exophthalmos is specific to Graves disease.
• Thyroid storm: abrupt, severe exacerbation of thyrotoxicosis, a medical emergency with risk of fatal cardiac arrhythmia, precipitated by infection, surgery, or abrupt withdrawal of anti-thyroid drugs.
• Apathetic hyperthyroidism: blunted classic features in the elderly, presenting as unexplained weight loss or worsening cardiac disease.
• Diagnosis: low serum TSH (most sensitive screening test) with elevated free T4 (or T3 in "T3 toxicosis"); radioiodine uptake scan differentiates Graves (diffuse uptake) from toxic adenoma (focal uptake) from thyroiditis (low uptake).