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Chapter 5-01: Anatomy and Physiology in Endocrinology

ENDOCRINE AXES

  • Organization: There are five parallel endocrine axes in the hypothalamic-anterior pituitary regulatory system (Figure 5.1)

  • Chapter organization: For the remainder of this chapter we will organize anatomy, physiology, pathophysiology, and treatments by endocrine axis

FIGURE 5.1

Endocrine axes in the hypothalamic–anterior pituitary regulatory system. There are five parallel endocrine axes in the hypothalamic–anterior pituitary regulatory system that control the biosynthesis and release of 1) thyroid hormone, 2) glucocorticoids, 3) the sex steroids estradiol and testosterone, 4) growth hormone, and 5) prolactin. (⊕) indicates stimulatory action and (⊖) indicates inhibitory action. Abbreviations: TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone; T4, thyroxine; T3, triiodothyronine; CRH, corticotropin-releasing hormone; VP, vasopressin; ACTH, adrenocorticotropic hormone; GnRH, gonadotropin-releasing hormone; FSH, follicle-stimulating hormone; LH, luteinizing hormone; GHRH, growth hormone-releasing hormone; GH, growth hormone; IGF-1, insulin-like growth factor-1; IGF-BP3, insulin-like growth factor binding protein-3.

Five flow charts show the endocrine axes that are involved in the hypothalamic - anterior pituitary regulatory system. The flow chart has five rows and each row are labeled as follows: hypothalamic hormone, anterior pituitary hormone, target organ, target organ hormone and function, respectively. The T R H stimulation and somatostatin inhibition is responsible for the metabolism function in thyroid is represented in the first column. The C R H and V P stimulation is responsible for stress response in adrenal cortex represented in the second column. The G N R H stimulation is responsible for the reproduction in gonads and testes. The G H R H stimulation and somatostatin inhibition is responsible for the growth in liver and tissues. The T R H stimulation and Dopamine inhibition is responsible for lactation in breast.
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PITUITARY

FIGURE 5.2

Hormones secreted by the pituitary gland and their functions. The pituitary is a small gland that is positioned in the base of the brain. The pituitary controls the function of most other endocrine glands, secreting the hormones shown here, which each act on distinct target organ(s) or tissues. As is shown, certain hormones are secreted by the anterior lobe, whereas other hormones are secreted by the posterior lobe.

An image shows of various hormones that are secreted by the pituitary gland and also their different functions. The pituitary gland shows a tube like structure at the top which is elongated to join bigger tube structure which has oval shaped structure at the bottom. The adrenal cortex is represented as a irregular Triangle, thyroid gland is represented as a double layered H shaped structure, the Kidneys are represented as C shaped structure, the breast are represent add as close bracket, the testis and ovary is are presented as oval shaped structure. cortical hormones, thyroid hormones, oxytocin, vasopressin and F S H are responsible for the functions of different glands and organs.
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Anterior Pituitary

Hypopituitarism

  • Etiologies:

    • - Impaired synthesis or secretion of one or more hormones; can be primary or secondary (process impairs hypothalamic hormones, with resultant pituitary failure)

    • - Primary: Traumatic (e.g., head injury), neoplastic (e.g., adenoma), infiltrative/inflammatory (e.g., sarcoidosis), vascular (e.g., sickle cell disease, pregnancy-related), infections (e.g., histoplasmosis, TB), development/structural (e.g., dysplasia)

    • - Secondary: Hypothalamic dysfunction or stalk interruption due to aforementioned processes

  • Clinical features: Depends on which hormones are lost and the extent and duration of deficiency

  • Hormone deficiencies:

    • - ↓ Growth hormone (GH): Fatigue, increased fat mass, decreased muscle mass, growth retardation (children). Clinically this is measured using GH suppression testing with oral glucose tolerance and finding an elevated IGF-1.

    • - ↓ Luteinizing hormone (LH)/follicle-stimulating hormone (FSH): Hypogonadotropic hypogonadism (infertility, amenorrhea, impotence)

    • - ↓ Thyroid-stimulating hormone (TSH): Hypothyroidism, growth retardation (children)

    • - ↓ Adrenocorticotropic hormone (ACTH): Secondary adrenal insufficiency (hypocortisolism; relative preservation of mineralocorticoid production)

    • - ↓ Prolactin: Inability to lactate

  • Diagnosis: Get pituitary MRI. Typically there are low levels of anterior pituitary hormones and thus low levels of target-organ hormones (e.g., low TSH, low free T4). Pearl: Hormone levels that are within normal lab ranges may be “inappropriately normal” based on the clinical context. For example, if free T4 is very low, then a normal TSH level would be inappropriately normal, since in this setting TSH should be elevated.

  • Treatment: Replace deficient hormones. Adrenal insufficiency and ...

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