Thyroid Gland and Thyroid Hormones

Question 1

The thyroid gland is located inferior to the ___, and is one of the largest endocrine glands. Each lobe is about ___cm long x ___cm wide, and weighs ~___g. The lobes are connected by an ___. The thyroid gland is highly ___.

Answer 1

larynx; 4; 2; 20; isthmus; vascular

Question 2

What is the internal structure of the follicle?

Answer 2

it is formed by spheres of epithelial cells and filled with colloid

Question 3

The ___ surface of the epithelial layer faces the colloid and the ___ surface faces the blood supply.

Answer 3

apical; basal

Question 4

The epithelial cells are also called ___ cells and they regulate the production of two ___-containing hormones.

Answer 4

follicle; iodine

Question 5

What does the lumen of the thyroid follicle contain?

Answer 5

thyroglobulin

Question 6

T/F. Thyroid hormones are peptide hormones that are stored in secretory vesicles.

Answer 6

False, thyroid hormones are LIPOPHILIC, and, thus, cannot be stored in secretory vesicles.

Question 7

Where is thyroid hormone synthesized and stored?

Answer 7

it is synthesized extracellularly in the colloid and stored there.

Question 8

How is iodide transported across the basal side of the follicle cell?

Answer 8

via a Na+/I- transporter

Question 9

How does intracellular iodide enter the colloid of the follicle?

Answer 9

it diffuses down its concentration gradient across the apical membrane

Question 10

The follicle cells synthesize a large, ___-rich protein called ___ and the enzyme ___ ___. Both proteins are ___ across the apical membrane into the colloid.

Answer 10

tyrosine; thyroglobulin; thyroid peroxidase; exocytosed

Question 11

T/F. The colloid is EXTRAcellular fluid.

Answer 11

True.

Question 12

How is iodide oxidized? Why is this a critical step?

Answer 12

Iodide is oxidized by thyroid peroxidase and linked to thyroglobulin. Binding I- to TG maintains the concentration gradient needed to continuously sequester I- in the colloid.

Question 13

If one I- is added to a ___ residue the resultant molecule is ___ but iodinating two sites produces ___.

Answer 13

tyrosine; monoiodotyrosine; diiodotyrosine

Question 14

How is tetra-iodothyronine (T4, or thyroxine) formed? Tri-iodothyronine (T3)?

Answer 14

DITs combine with either other DITs to form T4 or and MIT to form T3

Question 15

How is T3 and T4 stored?

Answer 15

attached to TG in the colloid

Question 16

Why is T3 and T4 unable to diffuse out of the cell?

Answer 16

because it is attached to TG

Question 17

What is the rate limiting step in thyroid hormone synthesis?

Answer 17

step 1. Iodide is transported across the basal side of the follicle cells via a Na+/I- transporter. This step depends on the availability of iodine in the environment/diet.

Question 18

How is the secretion of thyroid hormone stimulated?

Answer 18

by TSH, which is released from the anterior pituitary in response to TRH from the hypothalamus

Question 19

Where are TSH receptors located?

Answer 19

On the follicle cells. TSH also acts to increase synthetic activity of the follicle cells and stimulate hyperplasia

Question 20

In response to ___, droplets of colloid containing TG + T3/T4 are ___ into the follicle cells. The droplet fuses with a ___ containing enzymes that ___ T3 and T4 from the TG.

Answer 20

TSH; pinocytosed; lysosome; cleave

Question 21

T3 and T4 are released into the cytoplasm by lysosomal ___. They then diffuse into ___ and attach to binding proteins once inside the blood. List the three binding protiens they may bind.

Answer 21

hydrolysis; capillaries

Thyroid binding protein (TBP), Albumin, Prealbumin

Question 22

How is new TG formed?

Answer 22

Amino acids from the degraded TG are recycled into new TG

Question 23

Which form of thyroid hormone is secreted the most? Which form is more biologically active?

Answer 23

T4 (90%) is secreted the most but T3 (10%) is more active because it has a greater affinity to bind the receptor

Question 24

What happens to the secreted T4?

Answer 24

Most (80%) is converted to T3 in the liver and kidney

Question 25

When T4 is converted to T3, what happens to the extra iodide?

Answer 25

it is returned to the colloid.

Question 26

Why is it important that more T4 is secreted and not converted to T3 until it enters circulation?

Answer 26

This maximizes the concentration gradient for free iodide between the blood (high) and colloid (low) and helps insure that iodide is available for thyroid hormone synthesis. Otherwise the iodide would get secreted from the body.

Question 27

T/F. Receptors for TH are found on the membrane of cells.

Answer 27

False, they are found in the nuclei of most cells of the body.

Question 28

How many different receptor types are there?

Answer 28

Four. The alpha and beta forms are encoded by 2 different genes and type 1 and 2 result from alternative splicing of the transcript

Question 29

Expression of the receptor isoforms differs with respect to stage of development and target tissue. Give an example.

Answer 29

Alpha 1 is expressed during fetal development and Beta is expressed in neural tissue.

Question 30

___ occupied by TH will ___ and form a DNA binding protein that regulates gene transcription.

Answer 30

Receptors; dimerize

Question 31

What part of the TH receptor will stimulate or suppress expression?

Answer 31

The transactivation domain regulates the downstream transcription of proteins.

Question 32

How do TH receptors inhibit transcription?

Answer 32

Unbound receptor dimers can also bind DNA and typically inhibit transcription.

Question 33

What is the principle effect of TH?

Answer 33

to stimulate cellular metabolism

Question 34

TH increases ___/___-___ activity and synthesis to regulate the rate at which cells burn fuel/make ATP.

Answer 34

Na+/K+-ATPase

Question 35

Why does TH increase the synthesis of respiratory enzymes?

Answer 35

Because if one is burning more ATP, this will require that more enzymes are available to make it.

Question 36

How does TH increase substrate availability?

Answer 36

glycogenolysis, glycolysis, gluconeogenesis | acts on gut to increase absorption of nutrients

Question 37

TH increases cellular heat production when the rate of metabolism is increased. What is the mechanism?

Answer 37

TH increases uncoupling proteins in the ETC that take H+ and transport it back into the matrix instead of thru the ATPase. This produces heat.

Question 38

What is TH's effect on mitochondria?

Answer 38

it increases the number and or size

Question 39

TH upregulates ___-___ receptors that are critical to responses to activation of the sympathetic nervous system. It also increases sensitivities to ___ in both the endocrine and nervous system.

Answer 39

Beta-adrenergic; catecholamine

Question 40

TH regulates the production of ___ hormone, primarily in bone.

Answer 40

growth

Question 41

Why is TH important for CNS development and function?

Answer 41

At neuronal growth cones, precursors for axon/dendrite are stimulate to grow by TH It plays a role in the myelination of axons In adults, it maintains alertness and speed of reflexes

Question 42

What is the most common endocrine disease worldwide?

Answer 42

thyroid disease. Most conditions are due to hypothyroidism. Diabetes in the US

Question 43

T/F. An enlarged thyroid forms a goiter, but only in cases of hyperthyroidism.

Answer 43

False, An enlarged thyroid often forms a goiter, and some forms of BOTH hypo- and hyperthyroidism can lead to development of a goiter.

Question 44

Hypothyroidism is most often caused by a ___ defect of the thyroid gland. In 95% of cases, ___ deficiency (1 billion people worldwide) or ___ to the gland is involved.

Answer 44

primary; iodine; damage

Question 45

What happens without iodine?

Answer 45

There is insufficient production of TH leading to lack of negative feedback, which increases TRH and TSH secretion. This causes a growth in goiter

Question 46

How is iodine deficiency reversed?

Answer 46

by adding iodine to the diet. Iodized salt contains 1 NaI per 10,000 NaCl.

Question 47

What is the consequence of iodine deficiency during prenatal development?

Answer 47

moderate deficiency (10-15 point reduction in IQ) cretinism miscarriage or stillbirth Growth and neural development is impaired

Question 48

What is another name for the primary hypothyroidism disease known as Hashimoto's disease?

Answer 48

autoimmune thyroiditis

Question 49

How can the thyroid gland be damaged or destroyed?

Answer 49

surgical or radiation for thyroid cancers

Question 50

In the case of autoimmune thyroiditis, damaged or destroyed gland or dysfunction associated with illness (viral infection), the thyroid gland cannot make TH even if sufficient iodine is available. So, how is it treated?

Answer 50

the defects must be treated with exogenous TH. | Levothyroxine (synthyroid)

Question 51

Secondary effects arise from deficits in ___ or ___ synthesis and release, or receptor deficits. How are these treated?

Answer 51

TRH; TSH The thyroid function is fine, but the gland is not receiving the proper stimulatory input. Therefore, something that stimulates the action of TSH.

Question 52

What are the symptoms of mild hypothyroidism?

Answer 52

sensitivity to cold and slight weight gain. These cases are often not diagnosed.

Question 53

The symptoms of moderate hypothyroidism are due to the effects of reduced ___-___ receptors and the disruption of responses to ___ stimulation.

Answer 53

Beta-adrenergic; catecholaminergic The symptoms include fatigue, reduced bloodflow, changes in skin tone, sluggish gastrointestinal motility and mental function.

Question 54

What is the name of the condition that is associated with severe hypothyroidism?

Answer 54

myxedema

Question 55

Severe ___ is due to accumulation of ___ in the extracellular/intersitital fluid; typically most obvious in the ___.

Answer 55

bloating; glycosaminoglycans; face

Question 56

Hyperthyroidism or ___ results from having too ___ TH and is much ___ common than hypothyroidism.

Answer 56

thyrotoxicosis; much; less

Question 57

What are two primary defects that can cause hyperthyroidism?

Answer 57

1. thyroid tumors that produce TH without regulation by TSH | 2. inflammation of the gland that leads to the production of excess TH.

Question 58

___ disease is an autoimmune disease where ___ are produced against ___ and they can activate the ___ receptor. Therefore, there is no ___ regulation of thyroid function.

Answer 58

Graves; antibodies; TSH; TSH; feedback

Question 59

In Graves disease, ___ concentrations are high even though ___ (and ___) are low due to the feedback effects of increased ___.

Answer 59

TH; TSH; TRH; TH

Question 60

T/F. Graves disease is more common in women than in men.

Answer 60

True.

Question 61

___ ___ is hyperthyroidism that results from consuming excess thyroid hormone either in medication (hypo patient) or inappropriately processed meat.

Answer 61

Thyrotoxicosis factitia

Question 62

What do secondary defects that cause hyperthyroidism include?

Answer 62

tumors (in pituitary) that secrete TSH without responding to feedback control by TH.

Question 63

What can develop in cases of Graves disease that are caused by hyperstimulation of the TSH receptor even though TSH levels are low?

Answer 63

Goiter

Question 64

T/F. In hyperthyroidism, metabolic and nervous system responses include heat intolerance, weight loss, increased appetite, sweating, and hypersensitivity of catecholaminergic responses.

Answer 64

True.

Question 65

A general increase in activity of the sympathetic nervous system seen in hyperthyroidism is known as ___ ___.

Answer 65

thyroid storm

Question 66

What ocular symptoms are seen in hyperthyroidism patients?

Answer 66

exopthalmy, lid lag. and decreased visual acuity

Question 67

What two treatment options for hyperthryoid patients must be followed by replacement of TH to avoid hypothyroidism?

Answer 67

1. surgical removal of the gland | 2. destruction using radioactive I (which can also affect other organs that concentrate iodine - salivary glands)

Question 68

What is the function of antithyroid drugs?

Answer 68

they act to inhibit iodination of TYR block the release of TH ameliorate the effects of TH in peripheral tissues

Question 69

Why is TH not considered as a weight loss drug?

Answer 69

risk for heart attack b/c of beta-adrenergic side effects

Question 70

In hypothyroidism, ___ is associated with retarded tooth development and maxillary prognathism.

Answer 70

cretinism

Question 71

Which thyroid disease has a exaggerated response to narcotics and barbiturates, myxedema of the lips and tongue, and diminished cardiac and respiratory function due to lack of beta-adrenergic receptors?

Answer 71

hypothyroidism

Question 72

In hyperthyroidism, early eruption of teeth leads to ___.

Answer 72

malocclusion

Question 73

Which thyroid condition results in hypersensitivity to catecholaminergic drugs, osteoporosis in jaw bones due to increased bone resorption, susceptibility to thyroid storm?

Answer 73

hyperthyroidism