BMS 200 > Thyroid Physiology > Flashcards

Thyroid Physiology Flashcards

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1
Q

Where is the thyroid gland located?

A

Thyroid gland is located anterior to the trachea and below the larynx

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2
Q

what connects the two lobes of the thyroid gland?

A

thyroid isthmus

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3
Q

Some patients have a special shape lobe that is part of the thyroid, what is this called?

A

pyramidal lobe

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4
Q

where are the arteries of the thyroid from?

A
  • Superior thyroid artery from the external carotid
  • inferior thyroid artery from the subclavian artery
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5
Q

where do the veins of the thyroid drain into?

A

SVC or brachiocephalic trunk

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6
Q

thyroid begins to develop as ?

A

pit at the base of the tongue

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7
Q

what is obliterated soon after the thyroid gland is developed?

A

thyroglossal duct

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8
Q

what happens if the thyroglossal duct is not obliterated

A

small cyst from failure to eradicate the thyroglossal duct

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9
Q

The thyroid is surrounded by a thin capsule that attaches to deeper neck structures and the capsule sends…?

A

septa deep into the gland

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10
Q

Most of the thyroid is composed of ?

A

follicles

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11
Q

what is a follicle?

A

a spherical structure with (usually) a single layer of cuboidal epithelial cells on the outside and a large deposit of colloid on the inside

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12
Q

what is a colloid?

A

a proteinaceous substance mostly composed of thyroglobulin

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13
Q

what is the protein precursor of thyroid hormone?

A

thyroglobulin

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14
Q

what is the area in between follicles called?

A

parafollicular area

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15
Q

what is in the parafollicular area?

A

contains parafollicular cells that secrete calcitonin

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16
Q

What hormones do thyrocytes (follicular cells) secrete?

A

Thyroxine (T4) and Triiodothyronine (T3).

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17
Q

How are thyrocytes (follicular cells) arranged in the thyroid?

A

In spheres forming follicles, with a central colloid.

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18
Q

What do inactive thyroid follicular cells look like?

A

Flat cells with lots of colloid present.

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19
Q

What do active thyroid follicular cells look like?

A

Cuboidal or columnar cells; colloid uptake occurs via “reabsorption lacunae.”

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20
Q

What type of capillaries are found around thyroid follicles?

A

Fenestrated capillaries.

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21
Q

What cells are located outside the thyroid follicle?

A

C-cells (parafollicular cells).

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22
Q

What hormone do C-cells (parafollicular cells) produce?

A

Calcitonin

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23
Q

what is the main ingredients for thyroid hormone synthesis?

A

○ tyrosine and Iodine

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24
Q

which form (T4 or T3) is more active?

A

T3

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25
where is iodine absorbed?
small intestine
26
where is iodine mainly stored?
thyroid and some in the kidneys for excretion
27
where is iodine excreted?
Liver metabolizes thyroid hormones and releases some iodine into bile attached to the metabolites - 80% is excreted via kidneys
28
Where is the Na⁺/I⁻ Symporter (NIS) located on thyrocytes?
On the basolateral side (facing the capillaries).
29
What ions are transported by the Na⁺/I⁻ Symporter (NIS)?
2 Na⁺ and 1 I⁻ (iodide).
30
What enzyme helps maintain the Na⁺ gradient for the NIS to work?
Na⁺/K⁺ ATPase
31
What is the functional role of the Na⁺/I⁻ Symporter (NIS)?
Greatly concentrates iodide inside thyrocytes
32
What hormone increases NIS expression and retention in the membrane?
Thyroid Stimulating Hormone (TSH)
33
What is pendrin, and what does it do?
A Cl⁻/I⁻ exchanger that releases iodide into the colloid after transport through the thyrocyte
34
What is thyroglobulin?
Glycoprotein is a very large protein that contains lots of tyrosine residues
35
What synthesizes thyroglubulin?
○ Synthesized in the thyrocytes and released via exocytosis into colloid
36
what does the Thyroid Peroxidase Enzyme (TPO) do?
○ Oxidizes iodide and facilitates its binding to the 3C of tyrosine residues on the thyroglobulin
37
which carbons do the iodide attach to on the thyroglobulin?
Carbon-3 and carbon-5
38
What protein provides the tyrosine residues for thyroid hormone synthesis?
Thyroglobulin
39
What enzyme adds iodine to tyrosine residues on thyroglobulin?
Thyroid peroxidase (TPO)
40
What does TPO form when it adds one iodine to tyrosine?
Monoiodotyrosine (MIT).
41
What does TPO form when it adds two iodines to tyrosine?
Diiodotyrosine (DIT).
42
What thyroid hormone is formed from 2 DIT molecules?
Thyroxine (T4)
43
What thyroid hormone is formed from 1 DIT and 1 MIT molecule?
Triiodothyronine (T3)
44
What enzyme recycles iodide from unused MIT and DIT molecules?
Iodotyrosine deiodinase
45
How are T3 and T4 released from thyroglobulin inside follicular cells?
Thyroglobulin with attached T3/T4 is endocytosed, hydrolyzed, and T3/T4 are liberated.
46
What happens to thyroglobulin after T3 and T4 are released?
It is recycled back into the colloid
47
Are T4 and T3 hydrophilic or lipophilic?
They are relatively lipophilic
48
How are most thyroid hormones transported in blood?
Bound to plasma proteins (inactive)
49
Which form of thyroid hormones is biologically active?
The free form (unbound)
50
What is measured in lab tests for thyroid function: bound or free hormone?
Free T4 and T3
51
Name the main transport proteins for thyroid hormones.
- Albumin - transthyretin - thyroxine-binding globulin (TBG).
52
Which protein binds most of circulating T4?
Thyroxine-binding globulin (TBG)
53
Is T3 bound to TBG to the same extent as T4?
No, T3 binds TBG less; it is more active and used quickly
54
What happens if TBG increases rapidly?
Free T4 decreases → TSH increases
55
What happens if TBG decreases suddenly?
Free T4 increases → TSH decreases
56
In what conditions or states is TBG increased?
High estrogen, pregnancy, and certain medications
57
What is the half-life of thyroxine (T4)?
6–7 days
58
what enzyme remove iodine from the tyrosine residues in the periphery (tissues) and thyroid gland -> Allow for breakdown of thyroid hormones for excretion
Deiodinases (D1, D2, D3)
59
what enzyme facilitate interconversion of T4 into the more active T3, or the least active rT3
Deiodinases (D1, D2, D3)
60
Deiodinases (D1, D2, D3) contains what? which means it requires what?
Contain selenocysteine; requires selenium
61
Deiodinases can be influenced by variety of different factors:
- age - drugs - selenium deficiency - illness - diet
62
the thyroid stimulating hormones is composed of 2 subunits which are ?
alpha and beta
63
TSH-alpha is identical to the subunits of
LH, FSH and hCG
64
what can also activate TSH receptors
hCG (at high amounts)
65
the thyroid stimulating hormone secretion peak is when?
Pulsatile secretion with rise at 9pm, peak at midnight and decline after
66
What type of receptor is the TSH receptor?
A G-protein coupled receptor (Gs)
67
What second messenger pathway does the TSH receptor activate?
Phospholipase C pathway
68
What effect does TSH receptor activation have on iodide handling?
Increases iodide binding and activity of the Na⁺-I⁻ symporter (NIS).
69
How does TSH receptor activation increase T3 and T4 synthesis?
By enhancing TPO activity, endocytosis of iodinated thyroglobulin, and proteolysis of thyroglobulin
69
What effect does TSH receptor activation have on thyroglobulin?
Increases secretion of thyroglobulin into the colloid
70
How does TSH receptor activation affect thyroid blood flow?
It increases blood flow to the thyroid
71
What happens with chronic high stimulation of TSH receptors?
Hypertrophy or goiter formation
72
What do IGF-1 and EGF receptors do in the thyroid?
Promote thyroid gland growth
73
What cytokines oppose IGF-1 and EGF effects on the thyroid?
Interferon-gamma and TNF
74
How does cold exposure affect TSH secretion?
In infants: increases TSH; in adults: negligible effect
75
How does stress affect thyroid regulation?
Inhibits TRH secretion
76
How do cortisol and glucocorticoids affect thyroid regulation?
They inhibit TSH secretion
77
How do T3 and T4 enter target cells?
By diffusion or carrier-mediated transport
78
What happens to T4 once inside the cell?
It is deiodinated to T3
79
What does T3 do once inside the nucleus?
Binds thyroid hormone receptors (TRs), activating thyroid response elements (TREs) in DNA to influence gene expression.
80
Where are thyroid hormone receptors (TRs) located?
Inside the nucleus
81
What type of proteins are TRs?
Hormone-sensitive nuclear transcription factors
82
Which hormone binds more strongly to TRs, T3 or T4?
T3
83
What tissues are not affected by the calorigenic action of thyroid hormone?
Brain, pituitary, and adult reproductive organs
84
What enzyme activity is increased by thyroid hormones?
Na⁺/K⁺ ATPase
85
What are the major calorigenic effects of thyroid hormones?
↑ O₂ consumption, ↑ fatty acid metabolism, ↑ basal metabolic rate.
86
How can high thyroid hormone levels affect body weight and growth?
Small amounts stimulate growth; high amounts promote catabolism and weight loss if nutrient intake doesn’t match
87
How do thyroid hormones affect vitamin metabolism?
Increase requirement for all vitamins; needed for liver metabolism of carotene into vitamin A.
88
What are thyroid hormone effects on reproduction and lactation?
Facilitate normal menstrual cycles and allow for milk secretion.
89
What effect do thyroid hormones have on skin?
Support normal skin structure.
90
How do thyroid hormones interact with catecholamines?
They act synergistically with catecholamines.
91
How do beta-blockers influence thyroid hormone action?
They mildly block T4 to T3 conversion
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