Biological role of thyroid hormones

Main hormones and their effects

The thyroid gland produces iodinated compounds — thyroxine (T4) and triiodothyronine (T3), as well as the non-iodinated hormone thyrocalcitonin. Under their influence, the synthesis of enzymes, transport proteins, and structural proteins is activated within cells.

Main directions of thyroid hormone action:

• Metabolism: trigger an intense metabolic surge, stimulate lipolysis and glycogenolysis, and increase blood glucose concentration.
• Calorigenic effect: activate biological oxidation, increase oxygen consumption and heat production.
• Organ functions: increase heart rate and enhance central nervous system excitability.
• Morphogenic effect: ensure growth, development, and differentiation of various tissues, being especially critical for the formation of the skeletal and nervous systems in childhood.

Metabolism and transport in the body

Primarily T4 is synthesized in the thyroid gland. Extra-thyroidally, through deiodination, it is converted into highly active T3 or biologically inactive reverse rT3.

In the bloodstream, the hormones circulate predominantly bound to plasma proteins:

1. T4: more than 99% is protein-bound, with a half-life reaching about 7 days.
2. T3: binds to proteins to a significantly lesser extent, with a half-life of about 1 day.

Only the free hormone fraction is biologically active. At the target cell level, T4 enters the cell, is converted to T3, and binds to nuclear receptors, initiating the regulation of transcription of specific proteins.

Regulation of thyroid function

Control of gland activity is carried out through humoral, neural, and peripheral mechanisms:

• Humoral regulation: hypothalamic thyrotropin-releasing hormone (TRH) stimulates the adenohypophysis to release TSH (thyroid-stimulating hormone), which activates the synthesis and hydrolysis of thyroglobulin. A negative feedback mechanism operates: circulating thyroid hormones inhibit the secretion of TRH and TSH.
• Neural regulation: sympathetic nerve fibers enhance hormone-producing function, whereas stimulation of the vagus nerve inhibits it.
• Peripheral regulation: involves the enzymatic conversion of hormones into active or inactive forms directly in the tissues.

Iodine balance and consequences of disorders

For stable thyroxine synthesis, the body requires about 50 mg of iodine per year (approximately 1 mg per week). Iodization of table salt is performed to prevent deficiency.

Disorders of the organ lead to marked pathologies:

• Hyperfunction: causes nervousness, anxiety, irritability, muscle tremor, fatigue, and sleep disturbances.
• Hypofunction: in children leads to a severe condition — cretinism, while in adults it is accompanied by a sharp decrease in performance.

Class and exam questions

What else to study on this topic

• Mechanism of iodothyronine synthesis within thyroglobulin
• The role of thyroid peroxidase in the oxidation and iodination of tyrosine
• Pathways of inactivation and excretion of thyroid hormones via the liver and kidneys
• Involvement of phenylalanine and tyrosine in catecholamine synthesis
• Hereditary disorders of aromatic amino acid metabolism

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.