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Physiology > Thyroid Physiology > Flashcards

Thyroid Physiology Flashcards

1
Q

What are the three major steps of thyroid hormone synthesis?

A
  1. Uptake & concentration of iodide (I-) in the thyroid gland
  2. Oxidation & incorporation of I- onto tyrosine’s phenol ring
  3. Coupling of two iodinated tyrosines to form T4 or T3
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2
Q

What is the minimum daily requirement of iodine? What is the US daily average intake?

A

Min: 80 μg

Avg: 300-400 μg

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3
Q

What are some sources for iodine in the diet (8)? Best (*)?

A

Seaweed*

Iodized table salt (since 1924)

Tuna

Cod

Shrimp

Dairy

Eggs

Lima beans

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4
Q

Describe the Wolff-Chaikoff effect?

A

When iodine intake exceeds 2 mg/day, TH synthesis is suppressed for about 10 days. Synthesis resumes via the “escape phenomenon”

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5
Q

How does iodine enter the cells of the thyroid gland?

A

Active transport down electrochemical gradient by 2Na+/ I- symporter

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6
Q

What is the function of thyroglobulin?

A

Scaffolding glycoprotein that TH grows from; contains lots of tyrosine residues

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7
Q

What enzyme catalyzes the oxidation and incorporation of I- onto tyrosine, as well as coupling iodinated tyrosines together?

A

Thyroid peroxidase (TPO)

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8
Q

What are MIT and DIT?

A

Thyroid hormone (T4 & T3) precursors

MIT: 3-monoiodotyrosine

DIT: 3,5-diiodotyrosine

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9
Q

What are the compositions of T3 and T4?

A

T3: one DIT coupled to one MIT

T4: Two DITs coupled together

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10
Q

______ makes up 90% of total TH secreted, _____ makes up 9%, and _____ makes up 1%.

A

90% T4

9% T3

1% rT3

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11
Q

What are the functions of T4, T3, and rT3?

A

T4: prohormone

T3: biologically active TH

rT3: biologic dud; made for regulation

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12
Q

What enzymes convert T4 into T3 vs T4 into rT3?

A

T4 –> T3: 5’-monodeiodinase

T4 –> rT3: 5-monodeiodinase

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13
Q

When temperature decrease, T3 production ______. During fasting, T3 production _______. If you are starving and cold, which factor wins out?

A

Decrease temp: increase T3

Fasting: decreased T3 (more rT3)

Cold wins and 5’MD is activated

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14
Q

Does thyroxine-binding globulin (TBG) have a greater affinity for T3 or T4?

A

T4

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15
Q

How are T3 and T4 transported in the blood?

A

70% bound to TBG

  1. 5% bound to prealbumin or albumin
  2. 03% free T4
  3. 3% free T3
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16
Q

True or false: alterations in TBG do not disturb biological function if the thyroid gland is normal.

A

True

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17
Q

What is the major stimulator of thyroid hormone secretion? What does it stimulate exactly?

A

Thyroid-stimulating hormone (TSH)

Virtually every step in TH synthesis

Growth and vascularity of the thyroid gland

18
Q

What are the effects of feedback of T3 on the anterior pituitary? (3)

A

Represses transcription of TSH gene

Suppresses TSH release

Down-regulates TRH

19
Q

Does the TH receptor have a greater affinity for T3 or T4?

A

T3

20
Q

Why is levothyroxine chosen over liothyronine in the treatment of hypothyroidism?

A

Levothyroxine (synthetic T4) mimics a true physiological response

Lower cost

More stable (2yrs in dark bottle)

Lack of allergens

Easy lab measurement

Long half-life

Taken once daily

21
Q

What are T3’s major functions? (4)

A

Brain maturation

Bone growth

β-adrenergic effects

BMR increases

22
Q

How many thyroid hormone transporter proteins have been identified and how many are proven to have clinical significance?

A

16 identified

2 have clinical significance

23
Q

How does TH regulate basal metabolic rate (BMR)? (4)

A
  1. Increase number and size of mitochondria
  2. Increase O2 consumption
  3. Increase substrate use (synthesis & oxidation of fatty acids and glucose)
  4. Increased Na-K ATPase activity
24
Q

What does it mean for TH to be sympathomimetic?

A

Many actions of high thyroid hormone levels resemble increased SNS activity

25
Q

How does TH reinforce cardiovascular responses to Epi and NE?

A

Increases the number of β- and α1-adrenergic receptors

26
Q

Describe the permissive effect TH has for Epi and NE?

A

Increases lipolysis, glycogenolysis, and gluconeogenesis

27
Q

T3 stimulates the _____ growth, development, and maturation of bone. T3 also stimulates secretion of _____.

A

Linear

Growth hormone

28
Q

T3 is essential for the fetal development of the CNS. What is its function in the CNS in children and adults? (7)

A

Enhances speed and amplitude of reflexes

Wakefulness

Alertness

Responsiveness to stimuli

Awareness of hunger

Memory and learning capacity

Normal emotional tone

29
Q

Describe congenital hypothyroidism. What causes it? What are its symptoms?

A

Cause:

Deficient TH in utero

Symptoms:

Impaired growth of cerebellar and cerebral cortices, proliferation of axons and dendrites, and myelination

Irreversible brain damage if not treated immediately after birth

Decreases all metabolic processes

30
Q

What are some of the symptoms of hypothyroidism (there are a lot, but the (*) ones are most important)?

A

*Fatigue, lethargy

*Weight gain

Cold intolerance

Drowsiness

Thinning hair

*Dry skin

Prolonged reflex time

Depression

Mental slowness

Conspitation

Amenorrhea (heavy/irregular periods)

*Puffy face (myxedema)

*Goiter

31
Q

What is the difference between primary and secondary hypothyroidism?

A

Primary: thyroid failure

Secondary: pituitary or hypothalamic failure

32
Q

Describe Hashimoto’s thyroiditis.

A

Autoimmune; antibodies attack thyroid peroxidase, thyroid gland, or TSH receptors

Most common cause of hypothyroidism

33
Q

How are T4, T3, and TSH levels affected in primary hypothyroidism? Can a goiter present itself?

A

Decreased T4 and T3

Increased TSH

Goiter possible

34
Q

How are T4, T3, and TSH levels affected in secondary hypothyroidism? Can a goiter present itself?

A

Decreased T4 & T3

Decreased TSH

No goiter

35
Q

How are T4, T3, and TSH levels affected in iodine insufficiency? Can a goiter present itself?

A

Decreased T4 & T3

Increased TSH

Goiters HIGHLY likely

36
Q

What are some of the symptoms of hyperthyroidism (there are a lot, but ones marked (*) are most important)?

A

*Pretibial myxedema (Graves’ disease)

*Exophthalmos, lid retraction (Graves’ disease)

Heat intolerance (increased BMR)

Weight loss

*Nervousness/anxiety/irritability

Tremors

*Moist/warm skin

Increased defecation frequency

*Goiter

Bruit over thyroid

Fatigue

37
Q

What is the difference between primary and secondary hyperthyroidism?

A

Primary: Graves’ disease (autoimmune)

Secondary: Excess TSH or TRH

38
Q

What is the cause of Graves’ disease?

A

Autoimmune disease in which antibodies target thyroid receptors activate thyroid hormone synthesis (thyroid stimulating immunoglobulins or TSI)

39
Q

How are T4, T3, and TSH levels affected in Graves’ disease? Can a goiter present itself?

A

T3 & T4 increase

TSH decreases (TSI increases)

Goiter possible

40
Q

How are T4, T3, and TSH levels affected in secondary hyperthyroidism? Can a goiter present itself?

A

Increased T3 & T4

Increased TSH

Goiter possible

41
Q

How are T4, T3, and TSH levels affected by a hypersecreting thyroid tumor (toxic adenoma). Can a goiter present itself?

A

Increased T4 & T3

Decreased TSH

No goiter

Physiology (11 decks)

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