Morphology and cellular apparatus of the thyroid gland
Anatomoically, the thyroid gland consists of two lobes connected by an isthmus. The organ is located directly below the thyroid cartilage, encompassing the trachea on both sides.
The endocrine tissue of the gland is heterogeneous and represented by two functionally distinct cell types:
- Parafollicular K-cells (C-cells). These are the very structures responsible for producing calcitonin, a peptide hormone directly involved in the hormonal regulation of calcium metabolism.
- Thyrocytes. These are the primary cells forming the follicles of the gland. Their main task is the synthesis of iodine-containing thyroid hormones: triiodothyronine (T3) and thyroxine (T4). A colloid mass, based on the protein thyroglobulin, accumulates inside the follicles.
Multilevel regulation: hypothalamus and pituitary gland
Although C cells function autonomously, the bulk of the gland is under strict control of the hypothalamic–pituitary–thyroid axis. Hypothalamic neurosecretory cells release thyrotropin-releasing hormone (TRH). It acts on the adenohypophysis, prompting it to produce thyroid-stimulating hormone (TSH) and prolactin. In the central nervous system, TRH also acts as a neurotransmitter and neuromodulator.
The secretion of TSH (a glandotropic glycoprotein) follows circadian rhythms: its peak occurs in the hours before falling asleep. TSH exerts a powerful stimulating effect on the thyroid gland:
- Enhances blood supply to the organ.
- Activates the growth of follicular epithelium.
- Increases the uptake of iodides from the blood.
- Triggers all stages of iodine-containing hormone formation.
It is clinically important to remember that an excess of TSH inevitably leads to hyperfunction of the thyroid gland. Suppression of TSH production occurs during stress (pain, trauma, anesthesia), under the influence of glucocorticoids and somatostatin.
Mechanisms of synthesis, secretion, and feedback
The process of hormone creation is inextricably linked to iodine metabolism. Thyrocytes uptake iodides from the blood, after which they are oxidized to atomic iodine. This iodine attaches to tyrosine amino acid residues located on the thyroglobulin molecule (prohormone).
To release the finished substances into the blood, an endocytosis mechanism is triggered: droplets of colloid are engulfed by thyrocytes, where lysosomal enzymes break down thyroglobulin. As a result, free T3 and T4 pass through the basal membrane into the systemic circulation.
The main principle of humoral control here is negative feedback. A high level of circulating T3 and T4 automatically suppresses the release of TRH in the hypothalamus and TSH in the pituitary gland. The nervous system also contributes: the sympathetic division stimulates the gland's activity, while 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), which ensures a long half-life of about 7 days.
Upon reaching peripheral tissues, T4 undergoes deiodination. As a result of the cleavage of an iodine atom, it is converted either into highly active T3 (with a lifespan of about 1 day) or biologically inactive rT3 (reverse T3).
Mechanism of cellular action:
- Hormones bind to membrane receptors on target cells.
- They penetrate inside with the help of 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 spent hormones occurs in the liver via conjugation. Then they are excreted with bile into the intestinal lumen, partially reabsorbed, deiodinated in the kidneys, and finally leave the body via urine.
Briefly: answers to topical questions
Which hormones are the main regulators of calcium metabolism in the body?
The main regulators of calcium metabolism in the body are parathyroid hormone, calcitonin, and calcitriol (the active metabolite of vitamin D).
- Parathyroid hormone (PTH) increases the concentration of calcium ions in blood plasma.
- Calcitonin decreases the concentration of calcium ions in the blood.
- Calcitriol ($1,25(OH)_2D_3$) regulates calcium metabolism by increasing its intestinal absorption and renal reabsorption.
How does calcitonin affect the concentration of calcium in blood plasma?
Calcitonin lowers the concentration of ionized calcium in blood plasma.
Its physiological effects include:
- Suppression of bone tissue resorption.
- Facilitation of bone tissue mineralization.
- Promotion of calcium excretion from the body by reducing its renal reabsorption.
Which cells synthesize parathyroid hormone?
Parathyroid hormone is synthesized by the functional cells of the parathyroid glands—parathyroid cells (chief cells).
- Active chief cell exhibits signs of active synthesis and is responsible for parathyroid hormone secretion.
- Inactive chief cell performs a reserve function.
What are the main target organs for parathyroid hormone?
The main target organs for parathyroid hormone are bone tissue and the kidneys.
- In bone tissue, it stimulates calcium mobilization, leading to the release of calcium ions into the blood.
- In the kidneys, the hormone stimulates calcium reabsorption in the distal tubules and decreases phosphate reabsorption, as well as activating calcitriol synthesis.
How does parathyroid hormone affect bone tissue?
Parathyroid hormone stimulates the mobilization of calcium from bone tissue into the blood.
- Rapid effect: leaching of calcium from the matrix (bone fluid) into the blood.
- Slow effect: initiates a cascade via adenylate cyclase in osteoblasts, stimulates osteoclast function, accelerating bone matrix resorption.
- Pathology: hormone excess causes active bone resorption with its replacement by fibrous tissue (osteitis fibrosa).
In which organs does sequential hydroxylation occur to form active calcitriol?
Sequential enzymatic hydroxylation to form active calcitriol occurs in the liver and then in the kidneys.
- In the liver: cholecalciferol is hydroxylated at the 25th carbon atom to form 25-hydroxycholecalciferol (the main transport form).
- In the kidneys (proximal tubules): a second hydroxyl group is added under the action of the enzyme $1\alpha$-hydroxylase, forming the biologically active hormone calcitriol (1,25-dihydroxycholecalciferol).
How does calcitriol affect calcium absorption in the gastrointestinal tract?
Calcitriol activates and enhances the absorption of calcium (and phosphates) in the intestine.
Signal transduction mechanism:
- It penetrates the enterocyte membrane and binds to the intracellular VDR receptor.
- The complex is transported into the nucleus, binds to DNA, and stimulates the transcription of the calcium-binding protein gene (calbindin).
- It induces the formation of protein components of $Ca^{2+}$-ATPase.
How does parathyroid hormone secretion change in hypocalcemia?
In hypocalcemia (a decrease in blood calcium ion concentration), parathyroid hormone secretion is stimulated.
A calcium-sensing receptor is located on the membranes of parathyroid gland cells. A decrease in blood $Ca^{2+}$ levels activates hormone incretion, leading to the mobilization of calcium from bones, increased renal reabsorption, and activation of calcitriol synthesis to restore normocalcemia.
What effect does calcitonin have on tubular reabsorption in the kidneys?
Calcitonin decreases renal calcium reabsorption, thereby promoting the excretion of calcium from the body.
To remember the cell types of the thyroid gland, use this association: "T-cells (Thyrocytes) make T-hormones (T3/T4), while C-cells make C-hormone (Calcitonin)".
Class and exam questions
Which thyroid cells are involved in calcium regulation?
Parafollicular C-cells (also known as C-cells) are involved in the regulation of calcium metabolism. They synthesize the peptide hormone calcitonin.
What is thyroglobulin and what is its role?
Thyroglobulin is a protein that functions as a prohormone. It forms the basis of the colloid within the follicles and serves as a storage form for iodinated hormones.
How does the negative feedback principle operate in this system?
A high concentration of T3 and T4 hormones in the blood suppresses the production of thyrotropin-releasing hormone in the hypothalamus and thyroid-stimulating hormone in the pituitary gland.
Where is triiodothyronine (T3) formed?
Although some T3 is synthesized directly in the gland, the majority of it is formed peripherally in target cells by the deiodination of thyroxine (T4).
What else to study on this topic
- Mechanism of iodide oxidation to atomic iodine
- Role of lysosomal enzymes in thyroglobulin hydrolysis
- Process of T4 deiodination to T3 and rT3
- Specific carrier proteins for hormone transport across the cell membrane
- Conjugation of hormones in the liver and their enterohepatic circulation
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