Hormonal regulation: hypothalamus and anterior pituitary Thyroid - - PowerPoint PPT Presentation

Hormonal regulation: hypothalamus and anterior pituitary Thyroid gland histology Thyroid disorders

Signaling by regulatory molecules

• hormones travel via the circulatory system to their targets
• may travel long distances to reach their targets
• may have systemic effects (multiple targets)

Hormonal signaling: polar hormones

• most hormones are polar hormones
• most hormones are polypeptide hormones
• *receptors are often G-protein coupled receptors

(insulin, glucagon, amylin, secretin, CCK, GLP-1, GIP, vasopressin, gastrin, somatostatin, leptin, PYY, ghrelin, a-MSH, oxytocin, GnRH, FSH, LH, CRH, ACTH, TRH, TSH, prolactin, GHRH, GH, IGF-1, calcitonin, parathyroid hormone)

*

Hormonal signaling: nonpolar hormones

• longer time course of action
• hormonal action involves changing gene expression
• hormones modified by enzymes at target cells

Steroid hormones

estrogen progesterone testosterone aldosterone cortisol

Vitamin D (1,25(OH)2D) Thyroid hormones

triiodothyronine (T3) thyroxine (T4)

Nonpolar Hormones

The pituitary gland is situated below the hypothalamus, posterior to the optic chiasm

• ptic

chiasm stalk of pituitary gland

H

H: hypothalamus midsagittal view of brain enlarged view à Be able to identify stalk of pituitary gland and hypothalamus

Location of pituitary gland within the skull

Figure 8.213B, Gray’s Basic Anatomy

midsagittal view of nasal cavity showing pituitary gland inside the sella turcica and adjacent to the sphenoid sinus

head model in library

sphenoid sinus pituitary gland

Figure 7.8a, p. 208

There are two parts to the pituitary gland: anterior pituitary and posterior pituitary

A à anterior pituitary (adenohypophysis): glandular pituitary

A P

P à posterior pituitary (neurohypophysis): axons and terminals of neurohormonal cells with cell bodies in the hypothalamus hormones of the posterior pituitary: oxytocin and vasopressin

Hormones regulated by the hypothalamus and pituitary

hypothalamic hormones: stimulatory: GnRH, GHRH, TRH, CRH inhibitory: dopamine, somatostatin anterior pituitary hormones: FSH, LH, TSH, GH, ACTH, prolactin peripheral gland hormones: gonadal steroids (estrogen, progesterone, testosterone), thyroid hormones (T3 and T4), IGF-1, cortisol

Hormonal regulation of hormone release

for a nonpolar hormone: for a polar hormone:

• tropic hormone: a hormone that regulates hormone secretion by another endocrine cell
• tropic hormones may also stimulate growth of endocrine tissue

NOTE: some texts (such as Silverthorn) use the term “trophic hormones”

Hormone-releasing cells in the hypothalamus are neurosecretory cells

Connection between the hypothalamus and the anterior pituitary: hypophyseal portal vessels

Adapted from Figure 7.8b, p. 208

neurosecretory cells releasing hypothalamic hormones capillary bed at base of hypothalamus hypophyseal portal vessels capillary bed in anterior pituitary

• a portal vessel links two capillary beds
• the hypophyseal portal vessels provide a

restricted vascular link between the hypothalamus and the anterior pituitary

Connection between the hypothalamus and the anterior pituitary: hypophyseal portal vessels

Negative feedback regulation of hormone release for hormones regulated by the anterior pituitary

peripheral gland hormone binds to its receptors in the hypothalamus and anterior pituitary to inhibit secretion of tropic hormones à helps keep hormone secretion within the appropriate physiological range

Hormones regulated by the hypothalamus and pituitary

hypothalamic hormones: stimulatory: GnRH, GHRH, TRH, CRH inhibitory: dopamine, somatostatin anterior pituitary hormones: FSH, LH, TSH, GH, ACTH, prolactin peripheral gland hormones: gonadal steroids (estrogen, progesterone, testosterone, thyroid hormones (T3 and T4), IGF-1, cortisol

Regulatory sequences for hormones

reproductive sequence cortisol sequence thyroid sequence

The thyroid gland is located in the neck

Adapted from Figure 23.4, p. 735

Synthesis of thyroid hormones, T3 and T4

• follicle cells synthesize T3 and T4
• iodine is required for active T3, T4
• T3 and T4 are synthesized as part
• f a protein precursor called

thyroglogulin

Histology of the thyroid gland

thyroid follicles: spherical structures consisting of a simple cuboidal epithelium made up of follicle cells, surrounding a central region filled with thyroglobulin

follicle cells thyroglobulin

Thyroid follicles

T3 and T4 are nonpolar hormones

• T4 is main hormone produced by thyroid gland
• T3 is active thyroid hormone
• deiodinase in target tissues converts T4 to T3
• T3 acts by changing gene expression in target tissues

e.g. deiodinase

Thyroid hormone functions

• required for proper nervous system development
• required for proper growth

develpmental adult

• increases resting energy expenditure:
• increased synthesis and degradation
• increased expression of Na+/K+-

ATPase

• decreased efficiency of ATP synthesis

in mitochondria

• cardiac effects
• enhances sympathetic effects

Iodine deficiency causes hypothyroidism and may cause goiter

Figure 23.6a, p. 738 Figure 23.7b, p. 738

less active T3, T4 due to insufficient iodine in diet less feedback inhibition means more TSH secretion by anterior pituitary TSH stimulates growth in the thyroid gland to cause goiter goiter: enlarged thyroid gland

Clinical example: hypothyroidism

• diagnosis is by made by measuring TSH levels
• elevated TSH indicates hypothyroidism
• primary hypothyroidism can by caused by

autoimmune destruction of thyroid gland (Hashimoto’s thyroiditis)

• treatment: thyroid hormone replacement