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Physiology > Thyroid and HPT Axis > Flashcards

Thyroid and HPT Axis Flashcards

1
Q

What is the blood supply of the thyroid?

A

Superior and Inferior Thyroid Arteries

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2
Q

What is the cell type found in inactive follicles?

A

Squamous Epithelium

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3
Q

What is the cell type found in active follicles?

A

Cuboidal Epithelium

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4
Q

What is the follicle?

A

Epithelial cells surrounding lumen - lumen filled with colloid

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5
Q

What is the function of the C cells (parafollicular cells)?

A

Produce calcitonin and maintain the follicle

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6
Q

What are the 2 precursors of thyroid hormone?

A

Thyroglobulin (TG) and Iodide

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7
Q

What is the Wolf-Chaikoff effect?

A

Increases in iodide intake decrease gland transport and hormone synthesis and vice- versa.

Allows for constant iodine storage even with dietary changes.

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8
Q

How is the Wolf-Chaikoff effect used clinically?

A

Very high iodide doses are used to rapidly shut down thyroid hormone production in hyperthyroid patients

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9
Q

What are the different thyroid hormones?

A

T3
T4
rT3

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10
Q

What is T4?

A

Thyroxine.

Tightly bound to transport proteins in blood Binds to receptor with low affinity

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11
Q

What is T3?

A

Triiodothyronine.

Primary active form
Binds with high affinity, low capacity to receptor

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12
Q

What is rT3?

A

Reverse Triiodothyronine.

Inactive

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13
Q

What are the components of the HPT axis?

A

Hypothalamus - PVN
Pituitary - Thyrotropes
Thyroid

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14
Q

How is the HPT axis regulated?

A

Negative feedback by T4/T3 in hypothalamus.

Negative feedback in pituitary by intracellular T3

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15
Q

What does the polarization of the thyroid follicle refer to?

A

Different functions of the apical and basolateral membranes

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16
Q

What is the function of the apical follicle?

A

Apical surface is exposed to the lumen (colloid) and thyroid hormone synthesis occurs here with the iodination of TG

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17
Q

What is the function of the basolateral follicle?

A

Basolateral surface is exposed to the blood and is responsible for the uptake of iodine and the release of thyroid hormone

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18
Q

What occurs with iodide trapping?

A

TSH stimulates iodide (I-) trapping by increasing the activity of the NIS (sodium iodide symporter) co- transporter in the basal membrane of the follicular epithelial cell.

19
Q

What happens after iodide transported into the follicle?

A

I- transported to follicular lumen and oxidized by thyroid peroxidase (TPO) to form iodine (I).

20
Q

What is iodination?

A

Tyrosyl residues on TG will have iodine added to them

21
Q

What is the conjugation of iodinated tyrosines?

A

Conjugation of iodinated tyrosines to form T4 and T3-linked thyroglobulin (TG)

22
Q

What is the function of the drug carbimazole?

A

Inhibits thyroid peroxidase.

Iodide will not be oxidized into iodine.

23
Q

What happens of following conjugation of iodinated tyrosine?

A
  • Conjugated thyroglobulin with T4/T3 enters follicular epithelial cell.
  • Packaged in endosomes
  • TG, MIT, DIT, T4, T3 released from vesicle
  • T4/T3 secreted into circulation
24
Q

What is the function of the radioactive iodide uptake scan?

A

Measures iodide uptake and used to determine function of thyroid gland.

Normal is 25% in 24 hours
> 60% = hyperthyroid

25
Q

What is the function of Type I thyroid hormone conversion?

A

Outer and inner ring deiodinase
Liver, kidney, thyroid, skeletal muscle
Primary source of T3 in circulation

26
Q

What is the function of Type II thyroid hormone conversion?

A

Outer ring deiodinase

Brain, pituitary, placenta, cardiac muscle

27
Q

What is the function of Type III thyroid hormone conversion?

A

Inner ring deiodinase

Brain, placenta, skin

28
Q

What are the transport proteins for thyroid hormone?

A

Thyroxine-binding globulin
Transthyretin
Albumin

29
Q

What thyroid hormone is the most tightly bound in circulation?

A

T4

30
Q

What is the thyroid hormone receptor?

A

Nuclear receptor family

  • same as steroid hormones
  • forms heterodimers with
  • retinoic acid receptor (RXR)
31
Q

What is the receptor affinity of thyroid hormone receptor for T3 and T4?

A

High affinity, low capacity for T3

Low affinity for T4 – very little biological activity at physiological concentrations

32
Q

What are the functions of thyroid hormone?

A

Increases BMR (Stimulates hepatic gluconeogenesis/proteolysis/lipolysis)
Promotes CNS maturation
Increases beta-adrenergic receptors
Increases thermogenesis

33
Q

What is the cause of cretinism?

A

T3 is crucial for normal brain development

Cretinism is caused by iodine deficiency during development

34
Q

What are the effects of cretinism?

A

Short stature/impaired bone formation Mental retardation

Delayed motor development

35
Q

What is the effect of thyroid hormone on the heart?

A

T3 increases cardiac output

36
Q

Why are goiters formed in hyper and hypothyroidism?

A

Hyper - thyroid overstimulation

Hypo - no negative feedback on thyroid which will increase TSH levels and form goiter

37
Q

What is the cause of Graves Disease?

A

Autoimmune – antibodies stimulate TSH receptor (Long Acting-Thyroid Stimulator – LATS) which lead to increased T3/T4

38
Q

What are some symptoms of Graves Disease?

A

Diffuse symmetrical goiters with hyperthyroid
symptoms: tachycardia, opthalmopathy, irritability, hyperactivity, heat intolerance, weight loss, nervousness, muscle wasting

39
Q

What is the cause of Hashimoto’s Disease?

A

Autoimmune destruction of thyroid follicles that will destroy TPO and TG which causes hypothyroidism

40
Q

What are some symptoms of Hashimoto’s Disease?

A

Diffuse goiter with hypothyroid symptoms: lethargy, fatigue, hair loss, cold intolerance, brittle nails decreased appetite, weight gain

41
Q

What is the cause of thyroid storm?

A

Hyperthyroid disorder coupled with severe acute illness

42
Q

What are the symptoms of thyroid storm?

A 
High fever
Tachycardia
Altered mental status
Severe nausea, vomiting, diarrhea
Severe circulatory collapse ----- resulting in death
43
Q

What is the function of thyroid peroxidase (TPO)?

A

Converts iodide into iodine

Physiology (81 decks)

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