Morphology and Cellular Apparatus of the Thyroid Gland
Anatomically, the thyroid gland consists of two lobes connected by an isthmus. The organ is located directly below the thyroid cartilage, embracing the trachea on both sides.
Fthe endocrine tissue of the gland is heterogeneous and represented by two functionally distinct cell types:
- Parafollicular C cells. These structures are responsible for producing calcitonin, a peptide hormone directly involved in the hormonal regulation of calcium balance.
- Thyrocytes. These are the main cells forming the follicles of the gland. Their primary task is to synthesize iodine-containing thyroid hormones: triiodothyronine (T3) and thyroxine (T4). A colloidal mass accumulates inside the follicles, composed primarily of the protein thyroglobulin.
Multilevel Regulation: Hypothalamus and Pituitary Gland
Although C cells function autonomously, the bulk of the gland is under the strict control of the hypothalamic-pituitary-thyroid axis. Neurosecretory cells of the hypothalamus release thyrotropin-releasing hormone (TRH). TRH acts on the anterior pituitary (adenohypophysis), stimulating it to produce thyroid-stimulating hormone (TSH) and prolactin. In the central nervous system, TRH also acts as a neurotransmitter and neuromodulator.
TSH (a glycoprotein trophic hormone) secretion follows circadian rhythms, peaking in the hours before sleep. TSH exerts a powerful stimulatory effect on the thyroid gland:
- Enhances blood supply to the organ.
- Activates the growth of follicular epithelium.
- Increases iodide uptake from the blood.
- Triggers all stages of iodine-containing hormone synthesis.
Clinically, it is important to remember that excess TSH inevitably leads to thyroid hyperfunction. TSH suppression occurs during stress (pain, trauma, anesthesia), as well as under the influence of glucocorticoids and somatostatin.
Synthesis, Secretion, and Feedback Mechanisms
The hormone production process is inextricably linked with iodine metabolism. Thyrocytes take up iodides from the blood, after which they are oxidized to atomic iodine. This iodine is attached to tyrosine amino acid residues located on the thyroglobulin molecule (a prohormone).
To release the finished substances into the bloodstream, an endocytosis mechanism is triggered: colloid droplets are engulfed by thyrocytes, where lysosomal enzymes cleave thyroglobulin. As a result, free T3 and T4 exit through the basolateral membrane into the systemic circulation.
The main principle of humoral control here is negative feedback. High levels of circulating T3 and T4 automatically suppress TRH release in the hypothalamus and TSH release in the pituitary. The nervous system also contributes: the sympathetic division stimulates gland activity, whereas the parasympathetic division inhibits it.
Transport, Peripheral Metabolism, and Excretion
The thyroid gland secretes predominantly T4. In the blood, more than 99% of this hormone binds to plasma proteins (globulins and albumins), giving it a long half-life of approximately 7 days.
Upon reaching peripheral tissues, T4 undergoes deiodination. Through the removal of an iodine atom, it is converted either into highly active T3 (half-life of about 1 day) or biologically inactive reverse T3 (rT3).
Mechanism of Cellular Action:
- Hormones bind to cell-surface membrane receptors of target cells.
- They cross the membrane via specific transporters.
- Inside the cell, T4 is finally converted into T3.
- T3 binds to nuclear receptors, altering gene transcription and triggering the synthesis of specific proteins.
Inactivation of used hormones occurs in the liver via conjugation. They are then excreted via bile into the intestinal lumen, partially reabsorbed, deiodinated in the kidneys, and finally eliminated from the body in urine.