Thyroid Physiology Flashcards

1
Q

Within the thyroid, follicular cells secrete __ hormones whereas chief cells secrete ______ hormones.

A

Follicular cells = T3/T4 hormones;

Parafollicular (C-) cells = Calcitonin hormone

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2
Q

Out of the produced hormones from the follicular cells, ___ is the precursor and ___ is the bioactive hormone.

A
T4 prehormone (91%);
T3 active hormone (7%)
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3
Q

What is the significance of thyroid peroxidase (TPO) in the biosynthesis of thyroid hormone. Explain it in the 3 steps of thyroid hormone biosynthesis.

A

TPO first oxidizes tyrosine to the synthesized thyroglobulin protein. Secondly, TPO iodinizes the secreted thyroglobulin then lastly couples this protein to existing colloid in the follicular lumen.

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4
Q

What thyroid hormone is the most active form and from where is it formed?

A

T3 is the most active form that is made peripherally from circulating T4 hormone which is in abundance in the body.

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5
Q

What are the 3 main binding proteins for thyroid hormones?

A

Thyroxine-Binding Protein (TBG), Thransthyretin and Albumin are the 3 major binding proteins for T3/T4.

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6
Q

What effect does elevated levels of T3/T4 have on the Hypothalamic-Pituitary-Thyroid Axis (HPA)?

A

Rising thyroid hormone levels inhibit TRH and TSH production via negative feedback mechanism.

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7
Q

What effect does Thyroid stimulating hormone (TSH) have on metabolism and the thyroid?

A

This increases the basal metabolic rate (BMR), increases the proliferation of thyroid cells and increases vascularization within the gland. It stimulates ATP production over a long period of time.

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8
Q

Describe the pathogenesis of Hashimoto’s Thyroiditis.

A

This disease is hypothyroidism in which thyroid inhibiting immunoglobulins (TII antibodies) attach the thyroid gland leading to reduced TSH secretion and inflammation.

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9
Q

Describe the pathogenesis of Grave’s Disease.

A

This disease is associated with Hyperthyroidism. It involves thyroid stimulating immunoglobulins (TSI antibodies) that targets the thyroid gland resulting in high levels of T3/4 and TSI levels.

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10
Q

What is the correlation between TSH and thyroid hormones in Primary Hypothyroidism?

A

High TSH levels and low T3/4 hormones are indicated in Hashimoto’s Disease.

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11
Q

What is the correlation between TSH and thyroid hormones in Primary Hyperthyroidism?

A

Low TSH levels and high T3/4 hormones as seen in Grave’s Disease. TSH levels are low due to the negative feedback of high thyroid hormones.

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12
Q

The conversion of T4 to T3 hormone is catalyzed by the hormone: __________. The enzyme responsible for converting T4 to inactive rT3 is: _________.

A
5'-deiodinase = T4 to T3;
5-deiodinase = T4 to rT3
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13
Q

About 80% of circulating T4 hormone is bound to ________; whereas, the remaining 20% is bound to _____ and _____.

A

Thyroxine-binding protein (TBG); transthyretin and albumin

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14
Q

Thyroid hormone receptors alpha and beta are ______ in ______ tissues of the body. They are found on chromosomes ____ and ____, respectively.

A
TR-alpha = chromosome 17;
TR-beta = chromosome 3; both are co-expressed in most tissues of the body.
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15
Q

Describe the mechanism by which thyroid hormone regulates body temperature.

A

T4 stimulates txn factors that upregulate mRNA which produces respiratory enzymes that stimulates mitochondria. A boatload of ATP is generated to which a majority of it is used up by the cell. The remaining energy is emitted as heat that is used for body temperature.

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16
Q

List 4 signs and symptoms associated with a Hyper-thyroid disorder.

A

Elevated body temperature, low tolerance to environmental heat, profuse sweating, elevated BMR.

17
Q

Where do people get most of their dietary iodine? What makes iodine deficiency so consequential, in infants and adults?

A

Fortified bread and iodized salt are ways we receive iodine from diet. Iodine deficiency is associated with endemic cretinism (in infants) or the presence of a goiter (in adults).

18
Q

How would thyroid hormone treatment affect the development of a child with no functional thyroid tissue?

A

Thyroid supplement after 4.5 years of age may positively affect physical growth of such a child but has NO EFFECT on mental development. This is why testing for neonatal thyroid gland function is mandatory.

19
Q

Describe at least 5 symptoms that is linked with Hyperthyroidism. What disease(s) is it associated with?

A

Graves’ Disease. linked with an increased basal metabolic rate, increased heat production, osteoporosis, excitability, systolic HTN and exophthalmos (bulging eyes).

20
Q

Describe at least 5 symptoms that is linked with Hypothyroidism. What disease(s) is it associated with?

A

Hashimoto’s Thyroiditis (auto-immune) or Insufficient TSH secretion. Decreased BMR, weight gain, lethargy, mental slowness, diastolic HTN and decreased cardiac output.

21
Q

In what ways can hypothyroidism present in infants?

A

Cretinism, growth and mental retardation