17.8 thyroid gland

anatomy of thyroid gland

• Inferior to thyroid cartilage of larynx, anterior to trachea

• Left and right lobes

  • Connected at midline by isthmus

• Rich vascularization gives it reddish color

• Composed of microscopic follicles

thyroid follicles

numerous, microscopic, spherical structures that make up the thyroid

• Follicular cells—cuboidal epithelial cells that surround a central lumen, synthesize thyroglobulin (TGB)

  • Produce and release thyroid hormone (TH)

• Follicle lumen houses colloid—a viscous, protein-rich fluid

• Parafollicular cells— cells between follicles, make calcitonin

  • Hormone that decreases blood calcium levels

steps in regulation of thyroid hormone release

1. stimuli monitored by the hypothalamus

2. stimuli detected by hypo. the receptor

3. hypo. releases thyrotropin-releasing hormone (TRH)

4. anterior pit. responding to TRH releases TSH

5. TSH stimulates the thyroid gland to release TH in forms T3 and T4, lipid soluble and needs transport proteins

6. TH stimulates effectors

7. increased metabolic rate and release of nutrients into the blood to supply energy needed for higher metabolism

8. regulated by negative feedback

stimuli regulating release of TRH

1. TH levels low, needs to maintain a set point

• set point determined by:

  a. genetics

  b. aging

  c. environmental factors

effects of thyroid hormone TH

increases metabolism and initiates release of nutrients

• lipid soluble, needs transport proteins

• two forms: T3 and T4

  • T3: active form

  • T4: typically converted to T3 inside of cell, greater amount made at first

TH effect on all cells

TH increases metabolic rate of all cells

• increases their synthesis of sodium potassium pumps Na+/K+

  • converts ATP to hear

  • this rise in temp is called calorigenic

• heats body

• makes pumps

• gets pumps working

TH effect on liver

release of glucose into blood, increases:

• glycogenolysis (breakdown of glycogen into glucose)

• gluconeogenesis (synthesis of glucose from noncarb. resources)

• glycogenesis is inhibited (synthesis of glycogen from glucose)

TH effect on adipose CT

increases glycerol and fatty acids into blood as an alternative to nutrients for cellular respiration

• adipocytes respond to TH and increase lipolysis (breakdown of triglycerides into glycerol and fatty acids)

• inhibits lipogenesis (formation of triglycerides)

• saves glucose for brain (glucose sparing effect)

TH effect on respiratory and cardiovascular systems

• respiration rate increases to meet additional oxygen demands

• blood flow increases to deliver more nutrients and oxygen

• indirect on heart

  • up regulation of B-adrenergic receptors

  • increase heart rate and contraction

calcitonin regulation and effects

from parafollicular cells, reduces blood calcium from blood into bone

• activates when high calcium in blood

• kidneys to increase loss of Ca urine