Hypothalamic and pituitary hormone preparations

Anatomical features and system structure

The hypothalamus and pituitary gland form a single functional unit that serves as the central regulatory hub for the entire endocrine system. The anatomical structure of the pituitary gland is heterogeneous, which directly determines substantial differences in the synthesized hormones and, consequently, in the pharmacological agents used.

Traditionally, the pituitary gland is divided into two main lobes:

• Anterior lobe (adenohypophysis). This part contains specialized glandular cells. This is where the production of so-called "tropic" hormones occurs, whose task is to regulate the activity of peripheral endocrine glands or exert a direct effect on target tissues.
• Posterior lobe (neurohypophysis). Performs different functions in endocrine regulation.

Notably, some mammalian species also possess an additional intermediate lobe.

Three-tier hierarchy and feedback mechanism

The general principle of neurohumoral regulation is based on a strict three-tier interaction system:

1. Hypothalamic level. The hypothalamus produces specific releasing factors. These include liberins, which have a stimulating effect, and statins, which act as inhibitory factors.
2. Pituitary level. Under the direct action of liberins, the adenohypophysis begins to secrete tropic hormones.
3. Target organ level. Peripheral organs respond to the arrival of tropic hormones. The response is a change in their function or active secretion of their own effector hormones.

The key mechanism ensuring the stability of this system is the negative feedback principle (feedback mechanism). Its essence is as follows: a decrease in the concentration of peripheral gland hormones in the systemic circulation automatically stimulates the release of the corresponding releasing hormones and tropic factors. Conversely, an excess of peripheral hormones inhibits the function of the hypothalamus and pituitary gland. Pharmacological correction of endocrine disorders is based on the use of endogenous hormones, their synthetic drug analogs, or specific antagonists.

Pharmacology of the gonadotropic and thyroid axes

The sex gland regulation axis (gonadotropic axis) includes several levels. At the hypothalamic level, gonadotropin-releasing hormone (GnRH) is secreted. In clinical practice, its analog drug—gonadorelin—is used, which stimulates the secretion of gonadotropins by the pituitary gland. At the pituitary level, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are produced. Their pharmacological analogs are menopausal gonadotropin and chorionic gonadotropin. The target organs are the ovaries, where follicle development, as well as the production of estrogens and gestagens, occurs under the action of these drugs.

Special drugs are used to modulate the gonadotropic system:

• Danazol. This drug inhibits the production of gonadotropins, exerting an inhibitory effect on the pituitary gland.
• Clomifene. Acts by blocking estrogen receptors in the hypothalamus and pituitary gland. This artificially eliminates the negative feedback mechanism. As a result, a powerful stimulation of gonadotropin release occurs, leading to ovulation.

The thyroid regulation axis (thyroid axis) operates on a similar principle. The hypothalamus releases thyrotropin-releasing hormone (TRH), the analog of which is the drug protirelin. The pituitary gland responds by producing thyroid-stimulating hormone (TSH), represented in pharmacology by the drug thyrotropin. The target organ is the thyroid gland, where follicular growth, as well as the synthesis and secretion of thyroid hormones ($T_3$ and $T_4$), is stimulated.

Adrenal, prolactin, and somatotropic axes

• Adrenal regulation axis (adrenal axis). The hypothalamus controls the system via corticotropin-releasing hormone (CRH). At the pituitary level, adrenocorticotropic hormone (ACTH) is secreted, the drug equivalent of which is corticotropin. The target organ is the adrenal cortex, which responds by producing glucocorticosteroids (GCS).
• Lactation regulation (prolactin axis). Characterized by dual hypothalamic control. Stimulation is provided by lactotropin-releasing hormone (LHRH), while dopamine serves as the primary inhibitory factor. At the pituitary level, lactotropic hormone (LTH, prolactin) is produced, which acts on the mammary gland, inducing lactation. For the medical suppression of prolactin secretion, bromocriptine, a dopamine agonist drug, is used.
• Growth regulation axis (somatotropic axis). Also under dual hypothalamic control. Stimulation is carried out by somatotropin-releasing hormone (SRH), the analog of which is sermorelin. Inhibition is provided by somatostatin. Therapy utilizes octreotide, a long-acting synthetic analog of somatostatin. The pituitary gland releases growth hormone (GH, somatotropin), available as the drug somatropin. The targets are peripheral tissues, where the hormone exerts its anabolic action, stimulating tissue growth.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Anatomical and functional differences between the neurohypophysis and adenohypophysis.
• Mechanisms of action of glucocorticosteroids (GCS) on target organs.
• The effect of thyroid hormones ($T_3$ and $T_4$) on metabolism.
• The role of estrogens and gestagens in regulating the female reproductive system.

Other topics in the section "Hormone preparations, their synthetic substitutes, and antagonists"

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