Interaction with thyroid hormones
Although insulin is secreted by the pancreas, its metabolic effects are largely modulated by iodine-containing thyroid hormones (thyroid hormones). Thyroid hormones have a complex effect on carbohydrate metabolism, producing a hyperglycemic effect in the body.
- Accelerate the breakdown of cellular glycogen.
- Increase the rate of glucose absorption in the intestinal tract.
- Increase the rate of insulin inactivation in liver tissues.
The resulting hyperglycemia triggers a feedback mechanism that forces the pancreas to produce more insulin. Thus, thyroid hormones, on the one hand, stimulate insulin secretion and, on the other hand, act as its antagonists (counter-regulatory action). With prolonged imbalance, such strain can lead to the development of diabetes mellitus.
General effects of thyroid hormones on metabolism
In addition to carbohydrate metabolism, iodothyronines globally regulate cellular metabolism. They enhance tissue oxygen consumption and increase the body's baseline energy expenditure (basal metabolic rate).
- Protein metabolism: Normally, these hormones stimulate transcription in target cells, increase their membrane permeability to amino acids, and enhance protein synthesis. However, at excessive concentrations, catabolic processes are triggered, leading to a negative nitrogen balance.
- Lipid metabolism: They exert a powerful lipolytic effect, stimulating fat breakdown and accelerating fatty acid oxidation.
- Thermoregulation: They significantly increase heat production by activating cellular metabolism and enhancing the influence of the sympathetic nervous system.
Furthermore, thyroid hormones are critical for normal tissue growth and differentiation. They act synergistically with growth hormone (somatotropin), especially during skeletal growth, and are necessary for the adequate maturation of the central nervous system.
Thyroid dysfunction
Alterations in thyroid hormone secretion lead to systemic disruptions directly affecting metabolism and neural regulation.
- Hypothyroidism (hormone deficiency): Most commonly caused by iodine deficiency or impaired TSH production in the pituitary gland. It is characterized by a decrease in basal metabolic rate, a drop in body temperature, hypoglycemia, and suppression of mental and sympathetic activity.
- Hyperthyroidism (hormone excess): Manifests as tachycardia, elevated body temperature, hyperglycemia, and an increased basal metabolic rate. Patients exhibit excessive CNS excitability and signs of high sympathetic tone.
Role of calcitonin
An important regulator of mineral metabolism is calcitonin, a peptide hormone produced by parafollicular C-cells of the thyroid gland, as well as by thymus and lung tissues. Its receptors are located on cell membranes, and the signal is transmitted via second messengers (cAMP and cGMP).
The primary task of calcitonin is to lower ionized calcium (Ca²⁺) levels in the blood. The hormone inhibits the breakdown (resorption) of bone tissue, aiding in its mineralization, while simultaneously decreasing calcium reabsorption in the renal tubules and accelerating its excretion.
Sympathoadrenal system
Metabolic reactions, including those affecting carbohydrate metabolism, are closely linked to the sympathoadrenal system. Its hormones are catecholamines (adrenaline, noradrenaline, dopamine), which are sequentially synthesized from the amino acid tyrosine.
The bulk of catecholamines is produced in the adrenal medulla by chromaffin cells (modified sympathetic neurons), as well as in the ganglia of the autonomic nervous system. Catecholamines cause peripheral vasoconstriction and an increase in heart rate (HR). Notably, thyroid hormones can enhance the action of catecholamines by inhibiting the enzyme monoamine oxidase (which degrades them) and increasing the sensitivity of adrenergic receptors.
Regulation of the reproductive system
The gonads perform exocrine (gamete maturation) and endocrine (hormone synthesis) functions. This system is controlled by the hypothalamic-pituitary axis.
- Gonadotropin-releasing hormone (GnRH) is produced in the hypothalamus and released in pulses into the adenohypophysis, where it stimulates the production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In males, this process is inhibited by testosterone and inhibins, whereas in females it depends on the cycle phase: low estrogen levels stimulate FSH release, while high levels stimulate LH release.
- Male hormones: Androgens (testosterone) are synthesized by Leydig cells in the testes. Sertoli cells also participate in endocrine function by producing inhibins and converting some androgens into estrogens via aromatization.
- Female hormones: Estrogens are produced in ovarian follicles, and progesterone in the corpus luteum (its synthesis is additionally stimulated by pituitary prolactin).
Briefly: answers to topical questions
What hormones does the pancreas produce?
The endocrine part of the pancreas is represented by the cells of the islets of Langerhans; the main hormones are insulin and glucagon.
- Insulin is synthesized by $\beta$-cells of the islets of Langerhans; it stimulates glucose utilization and the synthesis of compounds for nutrient storage, lowering blood glucose levels.
- Glucagon is synthesized by $\alpha$-cells of the islets of Langerhans; it is an insulin antagonist and exerts hyperglycemic, lipolytic, and ketogenic effects.
- Somatostatin is synthesized by $\delta$-cells of the islets of Langerhans; it paracrine-inhibits the secretion of insulin and glucagon.
Which hormones exert a counter-regulatory (anti-insulin) effect?
Counter-regulatory effects are exerted by hormones that are insulin antagonists or that prevent the realization of its effects.
Counter-regulatory hormones include:
- adrenaline and other catecholamines;
- cortisol;
- iodothyronines / thyroid hormones;
- somatotropin / GH;
- glucagon.
An increase in counter-regulatory hormone levels is a hyperglycemic factor that prevents the realization of insulin effects.
Class and exam questions
How do thyroid hormones affect insulin secretion?
They cause hyperglycemia (by accelerating glycogen breakdown and glucose absorption), which, via a feedback mechanism, forces the pancreas to secrete insulin more actively.
Where does insulin inactivation take place?
One of the key organs where insulin inactivation takes place is the liver. Thyroid hormones can accelerate this process.
What happens to carbohydrate metabolism in hypothyroidism?
In hypothyroidism, overall metabolism slows down, which is usually accompanied by the development of hypoglycemia.
What else to study on this topic
- Mechanisms of intestinal glucose absorption
- Negative feedback in the endocrine system
- The role of monoamine oxidase in catecholamine metabolism
- The process of androgen aromatization in Sertoli cells
- Second messenger systems (cAMP and cGMP) in peptide hormone action
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