Thyroid Hyperfunction (Hyperthyroidism)
The primary clinical manifestation of hyperthyroidism is diffuse toxic goiter, also known as Graves' disease. This condition is characterized by a marked enlargement of the thyroid gland (goiter formation) and a sharp 2- to 5-fold increase in the plasma concentration of thyroid hormones—iodothyronines ($T_3$ and $T_4$)—compared to normal values. This hormonal excess leads to the development of thyrotoxicosis.
The clinical presentation of thyrotoxicosis includes a constellation of specific symptoms:
- Significantly increased basal metabolic rate.
- Tachycardia (elevated heart rate).
- Muscle weakness and prominent tremor.
- Progressive weight loss, which paradoxically occurs alongside an increased appetite.
- Excessive sweating and elevated body temperature.
- Exophthalmos (abnormal protrusion of the eyes).
At the metabolic level, both anabolism (tissue growth and differentiation) and catabolism are stimulated. However, catabolic processes strongly predominate: massive breakdown of carbohydrates, lipids, and proteins occurs. The biochemical marker of this accelerated protein catabolism is a negative nitrogen balance.
The principal triggers of hyperfunction include various tumors, inflammatory processes (thyroiditis), excessive intake of iodine or iodine-containing medications, and autoimmune responses.
Pathogenesis of Autoimmune Hyperthyroidism
The development of autoimmune hyperthyroidism involves a complex disruption of standard regulatory mechanisms.
- The body aberrantly produces autoantibodies directed against thyroid-stimulating hormone (TSH) receptors located on the membranes of thyroid follicular cells (thyrocytes).
- These autoantibodies, belonging to the IgG class, bind to the receptors and fully mimic the physiological stimulatory action of TSH.
- This results in continuous diffuse proliferation of the thyroid tissue and uncontrolled, excessive production of $T_3$ and $T_4$ hormones.
- Key feature: unlike physiological TSH secretion, the production of IgG autoantibodies is entirely exempt from negative feedback regulation.
- A distinct hormonal profile emerges: extremely high levels of circulating iodothyronines suppress normal pituitary function, causing the patient's serum TSH level to be significantly decreased.
Thyroid Hypofunction (Hypothyroidism)
Hypothyroidism develops as a result of thyroid hormone deficiency. The most common cause worldwide is iodine deficiency, leading to endemic goiter. Other causes include congenital genetic defects in enzymes required for iodothyronine synthesis (e.g., thyroperoxidase deficiency), complications from other disorders (involving structural damage to the hypothalamus, pituitary, or the thyroid gland itself), and autoimmune processes that produce antibodies against the protein thyroglobulin.
The disease presents in two main clinical forms depending on the patient's age:
- Cretinism — a severe form of hypofunction manifesting in early childhood, characterized by profound physical and intellectual developmental delays.
- Myxedema — a form of hypothyroidism developing in adults. Its hallmark feature is a specific "mucosal edema." The pathogenesis of this edema involves the abnormal, excessive accumulation of proteoglycans and water in the skin and interstitial tissues.
General symptoms accompanying hypothyroidism include persistent somnolence, a marked decrease in cold tolerance (patients complain of feeling constantly cold), stable weight gain, and a decreased core body temperature.