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Regulatory Physiology > Thyroid gland structure (week 2) > Flashcards

Thyroid gland structure (week 2) Flashcards

1
Q

what are c cells also known as and what do they secrete

A

parafollicular cells and they secrete calcitonin, a peptide hormone which can cause medullary carcinomas

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2
Q

what is a medullary carcinoma

A

this can happen when c cells become cancerous and grow out of control

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3
Q

what are thyroid epithelial cells also known as and what are they organised into

A

thyrocytes they are organised into spherical follicles

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4
Q

what hormones are secreted by the thyroid

A

T3, T4 these are the thyroid hormones

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5
Q

what is colloid

A

it is a glycoprotein

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6
Q

what do follicular cells secrete

A

thyroid hormone

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7
Q

what is 3,5,3’,5’-tetraiodothyronine also known as

A

thyroxine (T4)

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8
Q

what is 3,5,3’-triodothyronine known as

A

triiodothyronine (T3)

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9
Q

which is secreted in a greater amount T3 OR T4

A

T4 however T3 is more potent

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10
Q

how is T4 converted into T3

A

by a deiodinase which is an enzyme found in many peripheral tissues

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11
Q

what does T3 bind to

A

to T3 receptors that bind with high affinity (TR alpha and TR beta)

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12
Q

what are the three deiodinases

A

type 1,2,3

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13
Q

explain type 1 deiodinase

A

(low affinity, Km of microM) occurs in tissue with high blood flow and rapid exchanges with plasma,

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14
Q

explain type 2 deiodinase

A

(high affinity, Km of nM) expressed by glial cells in CNS, provides T3 even when free T4 falls to low levels

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15
Q

explain type 3 deiodinase

A

is the physiologic inactivator of thyroid hormones, catalyzing the inner ring deiodination of thyroxine (T(4)) to reverse triiodothyronine (rT(3)) and (T(3)) to 3, 3’-diiodothyronine (T(2)), both of which are biologically inactive.

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16
Q

what is thyroglobulin and what is its function

A

a protein made by the thyroid and it stores iodide

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17
Q

Thyroid hormone synthesis in thyrocytes

A

-Thyroglobulin (TG/TRG) is synthesized by thyroid epithelial cells, secreted into lumen of follicle. One TG contains
- Iodide taken from blood to colloid through Sodium-iodide symporter
-Enzyme thyroid peroxidase synthesises thyroid hormones and catalyses
1) iodination
2) coupling sequential reactions
-

18
Q

how many amino acids make up thyroglobulin

A

5500 amino 134 residues, makes up most of the follicular colloid

19
Q
A
20
Q

How much iodide is ingested and secreted daily

A

400 micrograms

21
Q

What are some minimum daily requirements of iodide needed in humans

A

150 micrograms for adults
90 to 120 micrograms for children
200 micrograms for pregnant women

22
Q

How much iodide is taken up daily by the thyroid and what is the thyroids overall iodide content

A

70-80 micrograms taken up daily

Total content is about 7500 micrograms

23
Q

What is the ratio of iodide stored in the form of hormones

A

100:1

24
Q

Physiological effects of thyroid hormone

A

Effects on cellular differentiation and development on the nervous system in particular

Effects on metabolic pathways and use of carbohydrates, lipids and proteins

25
Q

Explain how T3 and T4 promote accelerated metabolism

A

Increase oxygen consumption and increase heat production

Help regulate basal metabolic rate and body temperature

Increase carbohydrate, fat and protein turnover, ensuring that adequate cellular energy is available to support metabolically demanding activities

26
Q

Why are thyroid hormones important during development and growth

A

TH promote growth and stimulate maturation of central nervous system

Between week 11 and birth TH is essential

27
Q

Cardiovascular and respiratory effects TH

A

Alters the expression of Ryanodine Ca2+ channels in the sarcoplasmic reticulum, promoting Ca2+ release

Enhances the sensitivity and expression of adrenoreceptors to stimulation by noradrenaline

28
Q

Effects on basal membrane TH

A

Thyroid hormones increase the:
Basal rate of oxygen consumption
Heat production
Adjust heat loss through sweating and ventilation
Changes in body temperature parallel fluctuations in thyroid hormone availability

29
Q

Effects on autonomic nervous system and catecholamine TH

A

Increasing the number of beta- adrenergic receptors in heart muscle

Increasing the generation of intracellular second messengers, such cAMP

30
Q

TRH
TSH
meaning

A

Thyrotopic releasing hormone
Thyroid stimulating hormone
TSH stimulates synthesis of release of T3 and T4

31
Q

Hyperthyroidism effects

A

Increase HR
Increased cardiac contractility
Increased cardiac output

Decreased peripheral vascular resistance

Increased renal flow

32
Q

Hypothyroidism effects

A

Decreased HR
Decreased contractility
Decreased cardiac output

Increased peripheral vascular resistance

Decreased Renal blood flow

33
Q

How is hyperthyroidism caused

A

Autoimmune disease (Graves’ disease) autoantibodies (immunoglobulins) that stimulate thyrotropin receptors on thyroid gland follicle cells leading to continual stimulation of thyroid hormone synthesis.

Benign tumour of thyroid cells causing enlargement of gland and increased hormone secretion

Excessive secretion of TSH from a TSH-producing tumor

34
Q

How is hypothyroidism caused

A

Inflammatory/ autoimmune disease (hashimoto disease)_ antibodies attacking specific thyroid cellular components (e.g. thyroglobulin), causing gland damage

Defective hypothalamic and pituitary function causing insufficient thyrotropin secretion for normal stimulation of thyroid gland

Dietary iodine deficiency

35
Q

Graves’ disease

A

Highly characteristic symptom of weight loss despite increased intake of food, increased heat production causes excessive sweating, greater intake of water, trouble with heart rate
difficulty in swallowing or breathing due to compression of the esophagus or trachea by the enlarged thyroid gland (goitre)

Periorbital edema disagjosed by an elevated serum free and total t3 or t4 level

36
Q

Treatment of hyperthyroidism

A

Surgery
Partial or complete removal of gland obstructs neck veins and trachea, or if malaginant tumour present
Drugs
1) thioureylene (also called thiourea or thionamide) compounds
2) iodine-containing preparations
3) beta adrenoceptor

37
Q

Effects of thioureylenes

A

Orally active- inhibit thyroid hormone synthesis:
I) prevent iodination of tyrosine residues in thyroglobulin- probably by interfering with peroxidase enzyme action
Ii) prevent coupling reactions of monoiodo- and di-iodotyrosines

38
Q

Iodine-containing preparations

A

Radio iodine

Major method of treating hyperthyroidism

Taken orally as capsules of radioactive sodium iodide - radio iodine in blood accumulates in thyroid gland

Produces a-rays mainly b-particles

Used as single administration

39
Q

Beta adrenoreceptor antagonists

A

Propranolol usually

No direct effect on thyroid hormone synthesis, release etc

40
Q

treatment of hypothyroidism

A

hypothyroid individuals have weight gain despite poor appetite, cold intolerance, constipation and lethargy

synthetic T3, T4 used as replacement therapy

t4, first choice daily oral tablets, initial build-up to maximum effects takes several days because initially absorbed t4 molecules bind reversibly to blood plasma proteins; need these binding sites to become saturated

41
Q

what is the source and transport of T3 in the brain

A

t4 enters a glial cell and is converted to T3 by the deindinase. T3 exits the cell and is transported into a neuron via MCT8. Inside the neuron, it enters the nucleus, binding TR, or is inactivated to T2 by D3 deiodinase.

42
Q

MCT8 mutations

A

located on the x chromosome
males:
one copy
mutations result in allan-herndon-dudley syndrome.
affected males have abnormal plasma TH concentrations and neurological abnormalities.
females:
two copies

Regulatory Physiology (13 decks)

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