Anatomy and cellular composition
The thyroid gland is located on both sides of the trachea, just below the thyroid cartilage, and consists of two lobes connected by an isthmus.
The endocrine function of the organ is provided by two types of cells:
- Thyrocytes. They form follicles and are responsible for the production of iodine-containing hormones — T3 and T4. These are specifically referred to as "thyroid hormones" in the narrow sense.
- Parafollicular C cells. They synthesize the peptide hormone calcitonin.
Synthesis and secretion of thyroid hormones
Inside the follicles is a colloid, the basis of which is thyroglobulin, a prohormone and storage form of future active substances. The hormone production process directly depends on dietary iodine intake.
Stages of synthesis and secretion:
- Thyrocytes uptake iodides from the blood and synthesize thyroglobulin.
- Iodides are oxidized to atomic iodine.
- Atomic iodine attaches to tyrosine amino acid residues located on the surface of the thyroglobulin molecule.
- When necessary, droplets of colloid are returned to thyrocytes via endocytosis.
- Lysosomal enzymes hydrolyze thyroglobulin.
- The finished products (T3 and T4) are secreted through the basal membrane into the bloodstream.
Peripheral metabolism and mechanism of action
The gland itself produces predominantly T4 (thyroxine). However, outside the organ, in peripheral tissues, it undergoes deiodination. As a result of iodine cleavage, T4 is converted either into highly active T3 or biologically inactive rT3 (reverse T3).
How hormones act on target cells:
- First, they bind to membrane receptors and enter the cell via transporters.
- In the cytoplasm, T4 is deiodinated to T3.
- T3 enters the nucleus and binds to nuclear receptors.
- The resulting complex affects gene transcription.
- As a result, specific proteins are synthesized that provide the ultimate physiological effect.
Regulation of thyroid function
The function of the gland is controlled by a multilevel system where humoral regulation via a negative feedback mechanism plays the main role.
- The hypothalamus releases thyrotropin-releasing hormone (TRH). Its secretion is stimulated by severe cold and inhibited by high blood levels of T3 and T4. Interestingly, TRH is produced not only in the hypothalamus, but also in the amygdala and brainstem, where it acts as a neurotransmitter.
- The adenohypophysis releases thyroid-stimulating hormone (TSH) in response to TRH. TSH stimulates blood supply to the gland, growth of the follicular epithelium, iodine uptake, and all stages of hormone synthesis. Peak TSH secretion occurs in the hours before sleep. TSH production is suppressed by somatostatin, glucocorticoids, stress (pain, trauma, anesthesia), and thyroid hormones themselves.
- The nervous system plays an auxiliary role: the sympathetic division stimulates hormone synthesis, while the parasympathetic division inhibits it.
Inactivation and elimination
Used hormones are inactivated in the liver by conjugation. Then they are excreted with bile into the intestinal lumen. From there, some of the metabolites are reabsorbed into the blood, undergo deiodination in the kidneys, and are finally eliminated from the body in the urine.
To avoid confusing the hormones: T4 is the "Transporter" (circulates in the blood for 7 days, abundant), while T3 is the "Worker" (lives for 1 day, formed in tissues and actively acts right in the cell nucleus).
Class and exam questions
What is the difference between T3, T4, and rT3?
T4 (thyroxine) is the main product of the thyroid gland, serving as a reserve in the blood. T3 (triiodothyronine) is a highly active hormone formed from T4 in tissues. rT3 (reverse T3) is a biologically inactive form that is also produced from T4 in the periphery.
How exactly do thyroid hormones alter cell function?
They penetrate inside the cell, where T4 is converted into T3. Then T3 binds to nuclear receptors, directly affecting DNA transcription and triggering the synthesis of new specific proteins.
What will happen to TSH secretion during severe stress or trauma?
TSH secretion by the adenohypophysis will be inhibited. Stressful factors such as pain, trauma, or anesthesia, as well as glucocorticoids, suppress thyroid-stimulating hormone production.
What else to study on this topic
- Humoral regulation of functions and classification of biologically active substances.
- Hormonal regulation of calcium and phosphorus concentration (the role of calcitonin).
- Effector and glandotropic hormones of the adenohypophysis.
- Hypothalamic-pituitary system and mechanisms of stress development.
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