Thyroid Physiology Flashcards by Noah Wenger

What are the three major steps of thyroid hormone synthesis?

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

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

Min: 80 μg

Avg: 300-400 μg

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

Seaweed*

Iodized table salt (since 1924)

Tuna

Cod

Shrimp

Dairy

Eggs

Lima beans

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Describe the Wolff-Chaikoff effect?

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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How does iodine enter the cells of the thyroid gland?

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

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What is the function of thyroglobulin?

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

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

Thyroid peroxidase (TPO)

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What are MIT and DIT?

Thyroid hormone (T4 & T3) precursors

MIT: 3-monoiodotyrosine

DIT: 3,5-diiodotyrosine

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What are the compositions of T3 and T4?

T3: one DIT coupled to one MIT

T4: Two DITs coupled together

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

90% T4

9% T3

1% rT3

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

T4: prohormone

T3: biologically active TH

rT3: biologic dud; made for regulation

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What enzymes convert T4 into T3 vs T4 into rT3?

T4 –> T3: 5’-monodeiodinase

T4 –> rT3: 5-monodeiodinase

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

Decrease temp: increase T3

Fasting: decreased T3 (more rT3)

Cold wins and 5’MD is activated

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

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How are T3 and T4 transported in the blood?

70% bound to TBG

5% bound to prealbumin or albumin
03% free T4
3% free T3

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

True

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

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Thyroid-stimulating hormone (TSH)

Virtually every step in TH synthesis

Growth and vascularity of the thyroid gland

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

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Represses transcription of TSH gene

Suppresses TSH release

Down-regulates TRH

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

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

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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

What are T3’s major functions? (4)

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Brain maturation

Bone growth

β-adrenergic effects

BMR increases

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

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16 identified

2 have clinical significance

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

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Increase number and size of mitochondria
Increase O2 consumption
Increase substrate use (synthesis & oxidation of fatty acids and glucose)
Increased Na-K ATPase activity

What does it mean for TH to be sympathomimetic?

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Many actions of high thyroid hormone levels resemble increased SNS activity

How does TH reinforce cardiovascular responses to Epi and NE?

Increases the number of β- and α1-adrenergic receptors

Describe the permissive effect TH has for Epi and NE?

Increases lipolysis, glycogenolysis, and gluconeogenesis

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

Linear Growth hormone

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

Enhances speed and amplitude of reflexes Wakefulness Alertness Responsiveness to stimuli Awareness of hunger Memory and learning capacity Normal emotional tone

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

**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

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

\*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

What is the difference between primary and secondary hypothyroidism?

Primary: thyroid failure Secondary: pituitary or hypothalamic failure

Describe Hashimoto's thyroiditis.

Autoimmune; antibodies attack thyroid peroxidase, thyroid gland, or TSH receptors Most common cause of hypothyroidism

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

Decreased T4 and T3 Increased TSH Goiter possible

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

Decreased T4 & T3 Decreased TSH No goiter

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

Decreased T4 & T3 Increased TSH Goiters HIGHLY likely

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

\*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

What is the difference between primary and secondary hyperthyroidism?

Primary: Graves' disease (autoimmune) Secondary: Excess TSH or TRH

What is the cause of Graves' disease?

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

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

T3 & T4 increase TSH decreases (TSI increases) Goiter possible

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

Increased T3 & T4 Increased TSH Goiter possible

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

Increased T4 & T3 Decreased TSH No goiter

Thyroid Physiology Flashcards

(41 cards)