Hyperthyroidism - Pathophysiology

Definition and Key Distinction

Hyperthyroidism refers specifically to excessive thyroid gland function, resulting in overproduction of thyroid hormones (T3 and T4). This should be distinguished from thyrotoxicosis, which is the broader state of thyroid hormone excess from any source (including exogenous intake or gland destruction). - Harrison's Principles of Internal Medicine 22E

Normal Thyroid Hormone Axis (Background)

To understand how hyperthyroidism arises, the normal axis must be clear:

The hypothalamus releases TRH (thyrotropin-releasing hormone) → stimulates anterior pituitary thyrotrophs to secrete TSH
TSH binds its receptor on follicular cells (via Gs protein → adenylyl cyclase → cAMP), stimulating every step of thyroid hormone synthesis: I⁻ uptake via Na/I symporter, oxidation by thyroid peroxidase (TPO), organification into monoiodotyrosine (MIT) and diiodotyrosine (DIT), coupling to form T3 and T4, and secretion via proteolysis of thyroglobulin
Negative feedback: Free T3 (generated from T4 by 5'-deiodinase in pituitary thyrotrophs) down-regulates TRH receptors on thyrotrophs, suppressing further TSH release
99% of circulating thyroid hormones are protein-bound (to albumin, TBG); only free hormone is biologically active

Normal thyroid follicle histology - follicular cells surrounding colloid

Normal thyroid parenchyma - H&E stain showing follicular cells, colloid, and blood vessels (Sabiston Textbook of Surgery)

Causes of Hyperthyroidism

Category	Mechanism	TSH level
Graves' disease	TSH-receptor stimulating antibodies (TSI)	Low (feedback suppressed)
Toxic multinodular goiter	Autonomous nodule(s) secreting T3/T4	Low
Toxic adenoma	Single autonomously functioning nodule	Low
TSH-secreting pituitary adenoma	Excess TSH drives thyroid	High
Activating TSH-R mutation	Constitutive receptor activation	Low
McCune-Albright syndrome	Activating Gα mutation	Low
Struma ovarii	Ectopic thyroid tissue in ovarian teratoma	Low
Iodine excess (Jod-Basedow)	Iodine load in susceptible gland	Low

Harrison's Principles of Internal Medicine 22E, Costanzo Physiology 7th Edition

Graves' Disease - Primary Mechanism

Graves' disease accounts for 60-80% of thyrotoxicosis cases. It is an autoimmune disorder driven by:

1. Thyroid-Stimulating Immunoglobulins (TSI)

IgG autoantibodies against the TSH receptor (TSH-R)
TSI binds and activates the TSH receptor, mimicking TSH action - stimulating adenylyl cyclase → ↑cAMP → increased synthesis and secretion of T3/T4
Present in ~90% of Graves' patients; almost never seen in other thyroid autoimmune diseases
Simultaneously cause hypertrophy and hyperplasia of follicular cells (goiter)
Because circulating T3/T4 are elevated, endogenous TSH is suppressed by negative feedback - TSH levels are low in Graves' disease even though the gland is maximally stimulated

2. TSH-R Blocking Antibodies

A minority of patients also have blocking antibodies that compete at the TSH receptor
Coexistence of stimulating and blocking antibodies can produce fluctuating states - explaining occasional intercurrent hypothyroid episodes in Graves' patients

Robbins & Kumar Basic Pathology, Robbins, Cotran & Kumar Pathologic Basis of Disease

Genetic susceptibility: Concordance in monozygotic twins is 20-40% (vs <5% in dizygotic). Key susceptibility genes include HLA-DR alleles, CTLA-4, CD25, CD40, PTPN22, FCRL3, CD226, and the TSHR gene itself. - Harrison's Principles of Internal Medicine 22E

Environmental triggers: Stress (neuroendocrine effects on immune system), smoking (major risk factor for ophthalmopathy), sudden iodine increase, postpartum period, immune reconstitution (e.g. after HAART or alemtuzumab).

Molecular Mechanism of Thyroid Hormone Action

In target tissues, T4 is converted to the active T3 by 5'-iodinase (5'-deiodinase). T3 then enters the nucleus, binds to nuclear thyroid hormone receptors, and the T3-receptor complex binds thyroid-response elements on DNA → gene transcription → new protein synthesis.

Actions of thyroid hormones - mechanism diagram

Mechanism of action of thyroid hormones (Costanzo Physiology 7th Edition)

Key proteins induced by T3 include:

Na⁺-K⁺ ATPase → increased O₂ consumption, BMR, heat production
Myosin, β₁-adrenergic receptors, Ca²⁺ ATPase (cardiac) → increased heart rate and contractility
Metabolic enzymes (liver, adipose) → altered carbohydrate, fat, protein metabolism

Organ-System Pathophysiology in Hyperthyroidism

System	Mechanism	Clinical Features
Metabolic	↑Na⁺-K⁺ ATPase → ↑BMR, ↑O₂ consumption	Weight loss despite increased appetite, heat intolerance, sweating
Cardiovascular	Up-regulation of β₁-adrenergic receptors → ↑HR and contractility; catecholamine sensitization	Palpitations, tachycardia, ↑cardiac output, AF, wide pulse pressure
Neuromuscular	Catecholamine excess interaction, increased protein catabolism	Tremor, muscle weakness, nervousness, anxiety
Bone/growth	Accelerated bone turnover	Osteoporosis with prolonged disease
Metabolic (catabolic)	↑glycogenolysis, ↑gluconeogenesis, ↑lipolysis, net protein catabolism	Negative nitrogen balance, hyperglycemia tendency
GI	Increased gut motility	Diarrhea, increased frequency
Reproductive	Altered sex hormone-binding globulin	Menstrual irregularities in women

Catecholamine sensitization is a key amplifier: thyroid hormones up-regulate β₁-adrenergic receptors, so even normal circulating catecholamine levels produce exaggerated responses. This is why β-blockers are effective adjunct therapy. - Katzung's Basic and Clinical Pharmacology, 16th Edition

Graves' Disease - Extrathyroidal Manifestations

Ophthalmopathy (Exophthalmos)

TSH receptors are expressed not only in thyroid but also on orbital fibroblasts and fat cells
Activated CD4+ T cells secrete cytokines (including PDGF and TGF-β) that stimulate fibroblast proliferation and synthesis of hydrophilic glycosaminoglycans (hyaluronic acid, chondroitin sulfate)
This causes: (1) mononuclear cell infiltration of retro-orbital connective tissue, (2) edema and swelling of extraocular muscles, (3) GAG accumulation, (4) fatty infiltration (adipogenesis)
Net result: increased retro-orbital volume → protrusion of the eyeball (exophthalmos), lid lag, impaired extraocular movement

Pretibial Dermopathy (Pretibial Myxedema)

Deposition of glycosaminoglycans + lymphocytic infiltration in the dermis of the shins
Presents as scaly thickening and induration of skin

Robbins, Cotran & Kumar Pathologic Basis of Disease, Robbins & Kumar Basic Pathology

Gross and Histological Changes in Graves' Disease

Diffuse symmetric goiter: gland can weigh >80g, soft and brown-red on cut section
Histology: tall, crowded follicular epithelial cells forming small papillae projecting into follicular lumen (lacks fibrovascular cores - unlike papillary carcinoma)
Colloid: pale with scalloped margins (indicates active resorption)
Lymphoid infiltrates: T cells, B cells, plasma cells, often with germinal centers
Increased thyroidal blood flow - audible bruit may be present

TSH Levels as a Diagnostic Key

Cause	TSH level	Reasoning
Graves', toxic goiter, toxic adenoma, factitious	Low/suppressed	High T3/T4 → negative feedback suppresses pituitary TSH
TSH-secreting pituitary adenoma	High	Primary pituitary overproduction
Primary thyroid failure (e.g., Hashimoto's)	High	Loss of negative feedback on pituitary

Costanzo Physiology 7th Edition

Sources: Costanzo Physiology 7th Edition | Robbins & Kumar Basic Pathology | Robbins, Cotran & Kumar Pathologic Basis of Disease | Harrison's Principles of Internal Medicine 22E | Katzung's Basic and Clinical Pharmacology 16th Edition | Sabiston Textbook of Surgery

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