Stages of biosynthesis
The process of iodothyronine formation is a complex, multistep cascade of reactions taking place directly within the thyroid tissue.
- Iodide uptake. At the first stage, active transport of iodide from the bloodstream into the thyroid cells takes place. This process is carried out via symport together with sodium ions (Na+).
- Oxidation and iodination. The enzyme thyroid peroxidase (TPO) oxidizes the captured iodide. Then, iodination of tyrosine amino acid residues located within the thyroglobulin protein structure occurs. As a result, precursors are formed: monoiodotyrosine (MIT) and diiodotyrosine (DIT).
- Condensation. MIT and DIT molecules condense with each other, leading to the formation of full-fledged hormones — T3 and T4, which remain bound to thyroglobulin for now.
- Secretion. The colloid is captured by cells via endocytosis. Proteolysis of thyroglobulin occurs within lysosomes, after which free T3 and T4 molecules enter the bloodstream.
Regulation of secretion
Control over iodothyronine production is strictly subordinate to the hierarchical "Hypothalamus – Pituitary – Thyroid" axis and operates on the principle of negative feedback.
- Stimulatory pathway: The hypothalamus secretes thyrotropin-releasing hormone (TRH). This peptide acts on the pituitary gland, prompting it to release thyroid-stimulating hormone (TSH). In turn, TSH directly stimulates the thyroid gland to synthesize and release T3 and T4.
- Inhibitory pathway: When the concentration of iodothyronines in the blood becomes high, they begin to suppress the activity of higher regulatory centers. Excess T3 and T4 inhibit the production of both TSH in the pituitary and TRH in the hypothalamus, preventing excessive stimulation of the gland.
Physiological effects
By their chemical nature, iodothyronines are highly lipophilic. This property allows them to easily cross cell membranes. Their receptors have an intracellular (nuclear) localization. By binding to them, the hormones directly influence gene transcription.
Key physiological effects:
- Energy metabolism: Hormones stimulate the activity of Na+,K+-ATPase, which naturally leads to increased cellular oxygen consumption and an elevated basal metabolic rate.
- Thermoregulation: Iodothyronines ensure an adequate adaptive response of the body to cooling due to a significant increase in heat production.
- High-dose effects: An excess of hormones leads to the uncoupling of oxidative phosphorylation. In addition, the catabolism (breakdown) of proteins and fats is accelerated.
Clinical manifestations of pathologies
Thyroid dysfunction is divided into two opposing conditions, each with specific pathogenesis and symptoms.
Hypothyroidism (hormone deficiency):
- In adults, a severe form is called myxedema. Proteoglycans (glycosaminoglycans) and water accumulate in the connective tissue, causing characteristic mucous edema of the skin. Symptoms include a decrease in basal metabolic rate, cold intolerance, weight gain, bradycardia, and lethargy.
- If hypofunction occurs in early childhood, cretinism develops. It leads to irreversible mental and physical developmental delay, as well as impaired ossification processes.
Hyperthyroidism / Thyrotoxicosis (hormone excess):
- A classic example is Graves' disease (Basedow's disease). This is an autoimmune pathology characterized by the production of thyroid-stimulating antibodies (immunoglobulins). They bind to TSH receptors, mimicking its action and causing uncontrolled overproduction of iodothyronines.
- It manifests as an elevated basal metabolic rate, rapid weight loss, heat intolerance, tachycardia, tremor, and increased excitability.
Therapy monitoring
In the treatment of hypothyroidism with thyroxine preparations (L-T4), the most critical criterion for dose adequacy is the TSH level. Due to the negative feedback mechanism, taking an exogenous hormone should suppress TSH secretion to normal values. If TSH remains high, the dose is insufficient, whereas if it drops below normal, it is a sign of overdose.
Briefly: answers to topical questions
Which carrier proteins transport iodothyronines in the blood?
In the blood, iodothyronines (triiodothyronine and thyroxine) are transported by special carrier proteins — albumins and globulins.
Carrier proteins that transport thyroid hormones include:
- Albumin — a vital carrier protein that transports numerous substances, including thyroxine and triiodothyronine.
- Globulins — special proteins that bind and transport thyroid hormones in the bloodstream.
To remember pathology associations: "Hypo" — everything slows down (bradycardia, lethargy, weight gain, cold intolerance), "Hyper" — everything speeds up (tachycardia, tremor, weight loss, heat intolerance).
Class and exam questions
Where are iodothyronine receptors located?
Receptors for T3 and T4 are located intracellularly (in the nucleus). The hormones are lipophilic, cross the membrane, and directly regulate gene transcription.
How do high doses of iodothyronines affect mitochondria?
At high concentrations, they cause uncoupling of oxidative phosphorylation. This decreases ATP synthesis, accelerates the breakdown of proteins and fats, and energy is dissipated as heat.
What is the essence of the autoimmune mechanism in Graves' disease?
The body produces specific antibodies that bind to TSH receptors on thyroid cells. They act similarly to TSH itself, forcing the gland to continuously produce excess hormones.
Why is TSH specifically monitored during thyroxine therapy?
TSH is the most sensitive marker of the regulatory axis function. Via the feedback mechanism, it instantly reacts to blood T4 levels, indicating whether the prescribed drug dose is sufficient.
What else to study on this topic
- Structure and functions of thyroglobulin in the colloid
- The role of thyroid peroxidase and the mechanism of tyrosine iodination
- Sodium and iodide symport at the thyrocyte membrane
- The mechanism of proteoglycan accumulation in myxedema
- The effect of iodothyronines on Na+,K+-ATPase
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