Chapter 16 (Thyroid Hormone)

Question 1

Follicular Cells

Answer 1

• arranged in hollow spheres
• forms functional unit called a follicle
• lumen filled with colloid: serves as extracellular stroage sites for thyroid hormones
• produces two iodine containing hormones derived from an amino acid, tyrosine
  1. tetraiodothyronine (T4 or thyroxine
  2. triiodothyronine (T3)

Question 2

C Cells

Answer 2

• secretes peptide hormone calcitonin

Question 3

Synthesis of Thyroid Hormones

Answer 3

• all steps occur on thyroglobulin molecules within the colloid
• tyrosine-containing thyroglobulin is exported from follicular cells into colloid by exocytosis
• thyroid captures iodine from the blood and transfers it into the colloid by an iodine pump
• within the colloid, the iodine attaches to tyrosine
• coupling process occurs between iodinated tyrosine molecules to form thyroid hormones

Question 4

Storage

Answer 4

• thyroid hormones remain in colloid until they are split off and secreted
• usually enough thyroid hormone is stored to supply the body’s needs for several months

Question 5

Secretion

Answer 5

• follicular cells phagocytize thyroglobulin-laden colloid

- process frees T3 and T4 to diffuse across plasma membrane and into blood

Question 6

Functions of TH

Answer 6

• main determinant of basal metabolic rate
• influences synthesis and degradation of carbohydrate, fat and protein
• increases target-cell responsiveness to catecholamines
• increases heart rate and force of contraction
• essential for normal growth
• plays a crucial role in normal development of the nervous system

Question 7

Metabolic Rate and Heat Production

Answer 7

• thyroid hormones increases the body’s overall metabolic rate
• it’s an important regulator of O2, energy consumption and energy expenditure under resting conditions
• increased metabolic activities resulting in excess heat production (calorigenic effect)

Question 8

Intermediary Metabolism

Answer 8

• TH regulates/modulates the rate of specific reactions involved in fuel metabolism
• it influences both the synthesis and degradation of CHO, fats, and proteins. eg. the conversion of glucose to glycogen and vice versa
• a small amount of TH converts glycogen into glucose, but the reverse occurs with a large amount of TH
• adequate amounts of TH promotes protein synthesis essential for normal bodily growth
• hypersecretion of TH favors protein degradation

Question 9

Sympathomimetic Effects

Answer 9

• TH produces an action similar to sympathetic NS - sympathomimetic effect
• TH increases target cell responsiveness to catecholamines
• it exerts “permissiveness action” by causing proliferation of specific catecholamines

Question 10

Cardiovascular System Functions

Answer 10

• TH increases the heart’s responsiveness to circulatory catecholamines
• it increases heart rate and force of contraction to increase cardiac output
• causes vasodilation to carry out extra heat generated during calorigenic effects to the surface of the body for elimination to the environment

Question 11

Growth and Nervous System

Answer 11

• TH regulates growth through increasing GH secretion
• it stimulates GH secretion as well as promotes TH’s action in protein synthesis and skeletal growth
• TH deficient children experience stunted growth
• TH plays a role in the normal development of the nervous system in both children and adults
• TH deficiency from birth may cause CNS disorders

Question 12

Negative Feedback

Answer 12

• regulated by the negative feedback system between hypothalamic TRH, the anterior pituitary TSH and thyroid gland T3 and T4

Question 13

Hypothyroidism

Answer 13

caused by:

• thyroid gland fails to produce TH
• deficiency in TRH, TSH or oth
• inadequate dietary supply of iodine

symptoms:

• reduced metabolic rate
• displays a por tolerance of cold
• tendency to gain excessive weight, easily fatigued
• exhibits slow and weak pulse, slow reflexes and slow mental responsiveness
• mental effects include: diminished alertness, slow speech and poor memory
• develops edematous and myxedema (a puffy appearance of the face, hands, and feet)
• develop cretinism: dwarfism and mental retardation

Question 14

Hyperthyroidism

Answer 14

• causes Graves’ disease
• autoimmune disease
• body erroneously produces thyroid stimulating immunoglobulins (TSI)
• characterized by exophthalmos (bulging of the eyes out of the socket)

treatment

• surgical removal of a portion of the over secreting thyroid
• administration of radioactive idodine
• use of antithyroid drugs

symptoms:

• elevated basal metabolic rate
• increase of heat production, excessive perspiration and a poor tolerance to heat
• body weight falls
• causes a net degradation of CHO, fats and proteins, resulting in weakness
• various cardiovascular abnormalities (increased palpitations) caused by interacting effects of thyroid and catecholamines
• excessive mental alertness associated with irritability, tension, anxiety and mood swings

Question 15

Exoptalmus

Answer 15

• bulging eyes
• Graves disease causes the accumulation of water-retaining CHO behind the eyes
• excess fluid retention pushes the eyeballs forward so that the eyes bulge out of the bony orbits and the eyelids can’t be closed
• bulging eyes become dry, irritated and become prone to corneal ulceration