5.2 Hypothalamus-anterior pituitary axis

Hypothalamus-anterior pituitary axis

Hypothalamus-anterior pituitary axis

• Hypothalamic hormones control the secretion of anterior pituitary hormones → Anterior pituitary hormones control the secretion of hormones from the peripheral endocrine organs

Describe the (Growth hormone) GH axis including stimulation, inhibition, targets, and effects

• Hypothalamus:

  • GHRH (Growth Hormone-Releasing Hormone) → stimulates GH

  • Somatostatin (SS) → inhibits GH

• Anterior pituitary:

  • GH (Growth hormone / somatotropin)

• Target organs:

  • Muscles

  • Adipose tissue

  • Liver

• Effects:

  • Growth of muscles & bones

  • Stimulates IGF-1 production

Describe the CRH–ACTH pathway & its effects.

• Hypothalamus:

  • CRH (Corticotropin-Releasing Hormone)

• Anterior pituitary:

  • ACTH (Adrenocorticotropic hormone / corticotropin)

• Target organ:

  • Adrenal gland (cortex)

• Effect:

  • Secretion of glucocorticoids (cortisol)

Describe the TRH–TSH–thyroid hormone axis.

• Hypothalamus:

  • TRH (Thyrotropin-Releasing Hormone)

• Anterior pituitary:

  • TSH (Thyroid-Stimulating Hormone / thyrotropin)

• Target organ:

  • Thyroid gland

• Effect:

  • Release of:

    • T3 (triiodothyronine)

    • T4 (thyroxine)

Describe prolactin regulation & function.

• Hypothalamus:

  • Dopamine (DA) = prolactin-inhibiting hormone → inhibits PRL

  • PRH (prolactin-releasing hormone) → hypothetical / not clearly identified

• Anterior pituitary:

  • Prolactin (PRL)

• Target organ:

  • Mammary glands

• Effects:

  • Milk production

  • Breast development

→ Prolactin is mainly under inhibitory control (dopamine)

Describe the GnRH–LH/FSH axis including functions.

• Hypothalamus:

  • GnRH (Gonadotropin-Releasing Hormone)

• Anterior pituitary:

  • LH (Luteinizing hormone)

  • FSH (Follicle-stimulating hormone)

• Target organ:

  • Gonads (ovaries/testes)

• Effects:

  • Regulation of reproductive cycle

  • Secretion of steroid hormones:

    • Estrogen

    • Progesterone

    • Testosterone

Somatostatin inhibits

growth hormone

Dopamine inhibits

• prolactin

What are the target organs for each pituitary hormone?

• GH → Muscle, adipose tissue, liver

• ACTH → Adrenal gland

• TSH → Thyroid gland

• Prolactin → Mammary glands

• LH/FSH → Gonads

Hypothalamic hormones

• Dopamine

• TRH

• CRH

• Somatostatin

• GHRH

• GnRH

Anterior pituitary hormones

• Prolactin

• TSH

• ACTH

• GH

• FSH

• LH

Endocrine targets & the hormones they secrete

• Thyroid gland = Thyroid hormones

• Adrenal cortex = Cortisol

• Liver = Insulin like growth factors

• Endocrine cells of the gonads = Androgens, estrogens, progesteron

Nonendocrine targets

• Breast

• Many tissues

• Germ cells of the gonads

thyroid gland

• synthesizes & stores its hormones

thyroid gland produces

• Calcitonin

• Thyroid hormones (T3/T4)

Thyroid hormones are essential in

• development & metabolism regulation

thyroid gland contain

•  iodine (I)

&

• hydrophobic

Present in plasma in thyroid gland → two forms

• T3 → (Triiodothyronine)

• T4 → (Tretraiodotironine/ thyroxine)

T4 in plasma is

• abundant than T3 → but T3 is more active

The majority of T4 hormone is

• transformed to T3 inside the target cells

Most cells have receptors for thyroid hormones, so 

• T3 & T4 have effects throughout the body

T3 and T4 create a negative feedback loop with the

• hypothalamus & pituitary

Actions of thyroid hormones:

• Regulate basal metabolic rate & thermogenesis

• They regulate the processes of growth & development

Regulate basal metabolic rate & thermogenesis

• Increase the amount of energy consumed in a resting state, as well as the amount of heat produced by cells.

Thyroid hormones Stimulates

• Catabolism of carbohydrates and lipids

• Hepatic glucose synthesis

• Protein turnover

• Heart rate & ventilation rate

• Activity of Na+/K+ pump

• Oxygen consumption in mitochondria

• Heat production

They regulate the processes of growth & development is fundamental for

• The functioning of the central nervous system

Fundamental for the functioning of the central nervous system

• Embryonic development

• Adult CNS

Adult CNS

• Impaired T3 & T4 concentrations affect attention, memory, learning capacity, sleep/arousal cycles, & hunger sensations (hypo/hyper thyroidism).

Embryonic development

• Reduced T3/T4 levels during pregnancy result in reduced cortex & cerebellum formation → Severe cognitive deficits.

• Previously called cretinism, now CIDS.

Hypothyroidism

• Reduced T3,T4 secretion → Reduced negative feedback loop → Commonly accompanied by increased TRH and TSH levels.

Hypothyroidism causes

• Auto-immune attack (Hashimoto disease)

• Deficiency of iodine

• Injury to the thyroid

Hypothyroidism is due to

• low iodine

symptoms of Hypothyroidism

• Fatigue, slow cognitive processes & depression

• Reduced appetite but weight gain, low BMR

• Irregular or heavy periods

• Bradychardia

• Being sensitive to cold

• Goiter (excessive thyroid growth)

Hyperthyroidism

• Excessive T3 & T4 production

• Negative feedback to TRH & TSH.

Hyperthyroidism causes

• Graves disease: Immunoglobulins like TSH (TSI) in blood → Constant thyroid stimulation

• Thyroiditis: gland inflammation

• Excessive iodine consumption

• Tumor in the pituitary (TSH is increased)

Symptoms of Hyperthyroidism

• Increased heart rate

• Irritability, nervousness, mood swings

• Increased appetite but weight loss

• Excessive sweating

• Insomnia

• Goiter (excesive thyroid growth) & exophthalmos

Hyperthyroidism is due to

• Graves disease