Thyroid hyperfunction (hyperthyroidism)
The main clinical form of hyperthyroidism is diffuse toxic goiter, also known as Basedow's disease or Graves' disease. This condition is characterized by a marked enlargement of the gland itself (goiter formation) and a sharp 2- to 5-fold increase in the concentration of thyroid hormones—iodothyronines ($T_3$ and $T_4$)—in the blood plasma compared to normal. The consequence of such hormone excess is the development of thyrotoxicosis.
The clinical presentation of thyrotoxicosis includes a complex of specific symptoms:
- A significant increase in the basal metabolic rate.
- Tachycardia (increased heart rate).
- Muscle weakness and pronounced tremor.
- Progressive weight loss, which paradoxically occurs against the background of an increased appetite.
- Excessive sweating and elevated body temperature.
- Exophthalmos (specific proptosis).
At the metabolic level, there is a simultaneous stimulation of both anabolism (tissue growth and differentiation processes) and catabolism. However, catabolic processes are absolutely dominant: a large-scale breakdown of carbohydrates, lipids, and proteins occurs. The biochemical marker of such enhanced protein breakdown is a negative nitrogen balance.
The main triggers of hyperfunction include various tumors, inflammatory processes (thyroiditis), excessive intake of iodine or iodine-based preparations into the body, and autoimmune reactions.
Pathogenesis of autoimmune hyperthyroidism
The development of autoimmune hyperthyroidism is a complex process involving the disruption of standard regulatory mechanisms.
- The body begins to abnormally produce antibodies targeting thyrotropin (TSH) receptors located on the membranes of thyroid cells (thyrocytes).
- These antibodies, which are class IgG immunoglobulins, bind to the receptors and completely mimic the natural stimulating effect of TSH.
- As a result, continuous diffuse proliferation of the glandular tissue and uncontrolled, excessive production of $T_3$ and $T_4$ hormones occur.
- Key feature: unlike TSH production, the generation of IgG autoantibodies is not regulated by the physiological negative feedback mechanism.
- A specific hormonal profile emerges: extremely high levels of circulating iodothyronines suppress normal pituitary function, causing the patient's own blood TSH level to be significantly reduced.
Thyroid hypofunction (hypothyroidism)
Hypothyroidism develops due to a deficiency of thyroid hormones. The most common cause of this condition is iodine deficiency in the body, leading to endemic goiter formation. Other causes include congenital genetic defects in enzymes responsible for iodothyronine synthesis (e.g., thyroperoxidase deficiency), complications from other conditions (including damage to the hypothalamus, pituitary gland, or the thyroid gland itself), and autoimmune processes that produce antibodies against the thyroglobulin protein.
The disease manifests in two main clinical forms depending on the patient's age:
- Cretinism — a severe form of hypofunction occurring in early childhood. It manifests as profound physical and mental developmental delay in the child.
- Myxedema — a form of hypothyroidism developing in adults. A characteristic sign is specific "mucous edema". The pathogenesis of this edema lies in the excessive, pathological accumulation of proteoglycans and water in the skin.
General symptoms accompanying hypothyroidism include constant drowsiness, a noticeable decrease in cold tolerance (patients complain of feeling cold), steady weight gain, and a drop in overall body temperature.
Briefly: answers to topical questions
What specific biochemical reactions does the enzyme thyroperoxidase catalyze during hormone synthesis?
The enzyme thyroperoxidase catalyzes three key biochemical reactions during the synthesis of thyroid hormones.
- Oxidation — iodide ions are oxidized to active atomic iodine ($I^0$).
- Iodination (organification) — iodine atoms attach to tyrosine residues within thyroglobulin, resulting in the formation of monoiodotyrosines and diiodotyrosines.
- Condensation — coupling of iodinated tyrosine residues with each other: monoiodotyrosine and diiodotyrosine form triiodothyronine ($T_3$), while two diiodotyrosines form thyroxine ($T_4$).
Hyperthyroidism is a "fire" in the body: everything burns out (catabolism, leanness, sweating, tachycardia). Hypothyroidism is a "swamp": everything slows down (water accumulation, drowsiness, weight gain, feeling cold).
Class and exam questions
Which process dominates metabolism in thyrotoxicosis?
In thyrotoxicosis, catabolic processes dominate: active breakdown of proteins, lipids, and carbohydrates occurs. This is confirmed by the development of a negative nitrogen balance.
Why is the TSH level decreased in autoimmune hyperthyroidism?
Thyroid hormone synthesis is stimulated by antibodies (IgG) that are not subject to negative feedback. A massive excess of T3 and T4 is produced, which strongly suppresses pituitary function, causing endogenous TSH production to drop.
What is the mechanism of mucous edema formation in myxedema?
Edema forms due to a metabolic disorder in which proteoglycans excessively accumulate in the skin. These compounds actively bind and retain water in the tissues.
What else to study on this topic
- The biochemical role of the enzyme thyroperoxidase in the synthesis of iodothyronines.
- The detailed mechanism of IgG interaction with TSH receptors on the thyrocyte membrane.
- The relationship between negative nitrogen balance and protein catabolism in thyrotoxicosis.
- Molecular mechanisms of proteoglycan accumulation in the skin in myxedema.
- The role of iodine in the body and the mechanisms of endemic goiter development.
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