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(2020-21 BY2EN2) Endocrinology > Introduction to Thyroid Hormones > Flashcards

Introduction to Thyroid Hormones Flashcards

1
Q

State the different hormones of the thyroid

A
  • TRH -thyrotropin-releasing hormone
  • TSH -thyroid stimulating hormone
  • MIT -monoiodotyrosine
  • DIT -diiodotyrosine
  • T3 -triiodothyronine
  • T4 -thyroxine (tetraiodothyronine)
  • TG -Tg, thyroglobulin
  • TBG -thyroid binding globulin

• D1-D3 -deiodinases

• TR -THR, thyroid receptor

• RXR - retinoid X receptor (coregulator)

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2
Q

Label

A
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3
Q

Label the follicles of the thyroid gland

A
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4
Q

Draw the conversion of Tyrosine to Thyroxine (T4)

A
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5
Q

Draw the conversion of Tyrosine to Triiodothyronine (T3)

A
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6
Q

Describe the action of TSH on the Thyroid

A

•TSH binds the Thyroid stimulating hormone receptor (TSHR)

–major controller of thyroid cell function and growth

–mutations in TSHR may lead to several thyroid diseases, most commonly hyperthyroidism

–TSHR is divided into two subunits (α and β) by post-translational proteolysis and each subunit is linked to each other via disulfide bonds.

–GPCR – Gs/Gq

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7
Q

Thyroid Hormone Synthesis

A
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8
Q

Describe feedback control of thyroid hormones

A
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9
Q

Describe thyroid hormone secretion

A

Normal Daily Thyroid Secretion Rate:

  • T4 = 100 ug/day
  • T3 = 6 ug/day
  • ( ratio T4:T3 = 14:1 )
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10
Q
A
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11
Q

Explain conversion of T4 to T3

A
  • T3 has much greater biological activity than T4.
  • A large amount of T4 (25%) is converted to T3 in peripheral tissues (don’t confuse with 85% of T3 coming from T4)
  • This conversion takes place mainly in the liver and kidneys. The T3 formed is then released to the blood stream.
  • In addition to T3, an equal amount of “reverse T3” may also be formed. This has zero biological activity.

Advantages - The thyroid gland is capable of storing many weeks worth of thyroid hormone (coupled to thyroglobulin) therefore if no iodine is available for this period, thyroid hormone secretion will be maintained.

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12
Q

What are the key players of Deiodinases?

A

ORD = outer ring deiodination

IRD = inner ring deiodination (D1) Converts T4 to rT3

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13
Q
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14
Q

TRUE or FLASE: Hypothyroidism- D2 is upregulated

A

TRUE

D2 = upregulated (increases T3 in D2 expressing tissues)

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15
Q

TRUE or FLASE: Hypothyroidism- D3 is upregulated

A

FALSE

D3 = downregulated (decreases rT3 in D3 expressing tissues)

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16
Q

TRUE or FALSE: Hyperthyroidism-D1 is increased in thyroid gland itself (increased T3 released)

A

TRUE

17
Q

State the range of thyroid function tests

A

TSH: 0.4 –5.0 mU/L

Free T4 (thyroxine): 9.1 – 23.8 pM

Free T3 (triiodothyronine): 2.23 - 5.3 pM

18
Q

Explain the actions of thyroid hormones

A
  • Increase BMR eg increase oxygen consumption, energy expenditure, heat production
  • Increase turnover of metabolic fuels
  • Increase food intake
  • Increase responsiveness to catecholamines eg. tachycardia
  • Essential for growth, development and maintenance of neural integrity
19
Q

What are the metabolic action of thyroid hormones?

A
  • T3 stimulates carbohydrate absorption from the small intestine and increases fatty acid release from adipocytes (ie provide ATP).
  • ATP is used to support the activity of Na+/K+ -ATPases
  • As ATP is consumed by Na+/K+ -ATPases due to T3 stimulation, cellular stores of ATP must be maintained by increased fuel metabolism
  • This calorigenic action of T 3 represents a very significant fraction of the total heat produced each day in a typical person.
20
Q

What are the permissive actions of thyroid hormones?

A
  • T3 up-regulates β-adrenergic receptors in many tissues, notably the heart and nervous system.
  • Excess thyroid hormone concentration closely resemble some of the symptoms of excess epinephrine and norepinephrine
  • T3 potentiates the actions of the catecholamines, even though the latter are within normal concentrations
  • People with greater-than normal concentrations of T3 are often treated with beta blockers to alleviate excessive sympathetic activity
21
Q

What actions of tyroid of hormones releated to growth and development?

A
  • T3 is necessary for growth hormone production
  • Especially important for nervous system development
  • Absence of T3 during foetal life results in the syndrome called congenital hypothyroidism
  • This is most commonly caused by dietary iodine deficiency
  • T3 is also required for proper nerve and muscle reflexes and for normal cognition in adults
22
Q

What are some miscellaneous actions of thyroid hormons?

A
  • Prolactin production and secretion
  • Induces enzyme synthesis
  • Futile cycling
  • Protein synthesis
  • Bone growth
23
Q

Describe the expression/regulation of TH Receptors

A
  • TH receptors are found in multiple tissues of the body, but not in adult brain, spleen, testes, uterus, and the thyroid gland itself.
  • Thyroid hormone inhibits thyroid hormone receptor expression (TRE on THR genes).
24
Q

TRUE or FALSE: T3 is more biologically active than T4

A

TRUE

25
Q

Thyroid disorders: Hypothyroidism

A
  • Decreased activity
  • Iodine deficiency (simple goitre) – gland enlargement
  • Severe forms: cretinism in children, myxoedema in adults
  • Later stages of Hashimoto’s thyroiditis (autoimmune)

Signs, symptoms - decrease activity, tired, feel cold, slow mentally, decrease CO, wt gain, hair thinning/loss

26
Q

Thyrid disorders: Hyperthyroidism (Graves’ disease, thyrotoxicosis)

A
  • Increased activity
  • Mostly due to production of antibodies that stimulate excess secretion T3/T4
  • (thyroid-stimulating immunoglobulins). May cause modest goitre
  • Early stages of Hashimoto’s thyroiditis (autoimmune)
  • Signs, symptoms - increased activity, exopthalmos, weight loss despite increased appetite, palpitations, tremor, cold tolerance, muscle weakness, sweating.
27
Q

Draw a TRH stimulation test

A

A) 1° Hypothyroidism

B) Central Hypothyroidism

C) Euthyroid

D) 1° Thyrotoxicosis

28
Q

What is the treatment for Thyroid Disease(Hypothyroidism)?

A

Thyroxine sodium (levothyroxine) 25-200 mg per day

29
Q

What is the treatment for Thyroid Disease(Hyperthyroidism)?

A
  • Carbimazole 15-40 mg
  • Propylthiouracil 50 mg
  • Radioactive iodine 131
  • Partial thyroidectomy
  • Symptommatic relief

(2020-21 BY2EN2) Endocrinology (22 decks)

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