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Physiology > Thyroid Hormones > Flashcards

Thyroid Hormones Flashcards

1
Q

Hypothalamic TRH > Pituitary TSH > Thyroid gland produces thyroxin (T4) and triiodothyronine (T3)

Both T4 and T3 have the effect of _____, adjusted to alteration in ____, ___, ___, ___

A

T4 & T3 increase O2 consumption & metabolism

Adjusted in alteration in heat production, energy need, caloric supply, and environmental temperature

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2
Q

Describe the structure of a follicle.

A

Cuboidal endocrine cells surrounded by basement membrane forms single-layered circular follicles, the lumina of which contain thyroid hormones stored as colloid material

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3
Q

Thyroid gland also contains ___ that secrete ___.

A

parafollicular / C - cells

Secretes calcitonin

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4
Q

The stimulation of thyroid by ___ causes follicular cells to become more active at what?

A

TSH –> follicular cells become more active at

  • Iodine uptake
  • TH production
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5
Q

Iodothyronines

A

Secretory products of the thyroid glands

Iodine incorporated into 2 tyrosines

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6
Q

The major secretory product / iodothyronine of thyroid glands is ___, also known as ___.

A

3, 5, 3, 5 tetraiodothyronine AKA thyroxin (T4)

Functions largely as a circulating prohormone

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7
Q

The minor iodothyronine secreted is ___ AKA ___, which functions to ___

A

3, 5, 3 triiodothyronine AKA triiodothyronine (T3)

Provides almsot all thyroid hormone activity in target cells; produced in various tissues from the circulating prohormone T4

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8
Q

When less thyrhoid hormone action is needed, what happens to prohormone T4?

A

It’s turned into revers T3 (rT3), which is inactive and has unknown hormonal action

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9
Q

Thyroid hormone synthesis steps

A
  1. UPTAKE AND CONC OF IODIDE in the gland
    1. Actively transported via Na-I cotransporter
    2. Iodide trap: process that maintains [iodide]gland >> [iodide]plasma
  2. OXIDATION & INCORPORATION OF IODIDE
    1. Thyroid peroxidase incorporates iodide in Tyr of thyroglobulin –> monoiodotyrosine (MIT) and diodotyrosine (DIT)
  3. COUPLING OF 2 IODINATED TYROSINES
    1. Thyroid peroxidase couples
      1. two DITs –> T4
      2. MIT + DIT –> T3
    2. T4:T3 is usually 10:1, but restricted iodide availability leads to increased T3 formation
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10
Q

Thyroid peroxidase

A

Uses H2O2 to oxidize iodine

Iodinates the tyrosines on thyroglobulin (protein w the Tyrs)

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11
Q

Thyroid hormones within ___ are cleaved by ____ and go through the basement membrane to enter the capillary blood.

A

Thyroglobulin cleaved by lysosomal enzymes to release T3 & T4, which will go through the basement membrane, leave the cells and enter into the capillary blood.

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12
Q

Deiodinase

A

The MIT and DIT molecules are rapidly deiodinated by enzyme deiodinase and iodide is reutilized for new hormone synthesis.

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13
Q

Where are iodinated thyroglobulins store?

How do they get transferred into endocrine cells?

Do they leave the endocrine cell?

A

Stored as colloid in follicle

Transferred from lumen of follicle into endocrine cells by endocytosis, but intact thyroglobulin doesn’t leave the cell.

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14
Q

Wha treactions occur within the endocrine cell?

A

Iodide traps

Thyroglobin synthesis

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15
Q

What reactions occur at the interface between the cell membrane and the colloid?

A

Iodination rxn

Coupling rxn

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16
Q

What reactions occur after thyroglobulin’s reabsorption from the colloid?

A

Cleavage by lysosomal enzymes to release T3 & T4

–> secretion of T3 & T4

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17
Q

Thiouracil drugs

A

Blocks enzyme peroxidase to treat thyroid hyperfunction

18
Q

Iodide itself can be used to treat ___thyroidism

A

hyperthyroidism

19
Q

Negative feedback of thyroid hormones

A

T4 & T3 inhibit synthesis of both TSH & TRH

T3 block TRH’s effect and suppresses its release

20
Q

TRH or TSH:

Which is a tripeptide?

Which is a glycoprotein?

A

TRH = tripeptide

TSH = glycoprotein composed of two peptide subunits

21
Q

Synthesis & action of TRH

A

Cut from a precursor tetrapeptide to a tripeptide TRH

  1. TRH binds its own receptor
  2. Increases in Ca2+ and IP3 –> stimulate release of TSH from anterior pituitary
  3. Downregualtes/desensitizes its own receptor
22
Q

TSH / Thyrotropin functions

A

Stimulates all aspects of metabolism & growth of thyroid cells as well as

  • iodide trapping
  • thyroid hormone synthesis & secretion
  • colloidal endocytosis
23
Q

Iodide deficiency has what impact on TSH secretion?

A

Increases TSH secretion because thyroid hormone isn’t being synthesized

24
Q

Goiter

A

enlarged thyroid gland due to TSH hypersecretion

Ex) from iodide deficiency -> cant make thyroid hormone ->increase TSH

25
Q

What inhibits TSH secretion?

A
  • T3 suppresses TSH release; represses TSH gene expression; downregulates TSH receptors
  • Dopamine
  • Somatostatin
  • Cortisol
  • Growth hormone
26
Q

T4 and T3 are bound to what proteins while circulating?

A
  • Thyroxin-binding globulin (TBG) binds ~20% of T4 and T3
  • Transthyretin (thryoxin-binding parvalbumin)
  • albumin
27
Q

The __rity of total T4 and T3 are in the free state

28
Q

What is the function of TBG & transthyretin?

A

To create a circulating reservoir of T4 –> provides buffering capacity against cute changes in thyroid gland function

29
Q

Sustianed changes in the daily T4 supply from thyroid disease can lead to

A

sustained changes in both the bound and free reactions

30
Q

Acute hepatic disease, pregnancies, estrogen therapy, or kidney disease can change ___ levels, which will disturb the ratio of free to bound T4

A

serum TBG level

31
Q

Thyroid hormone replacement therapy is carried out with which thyrhoid hormone?

A

T4 (less hormonally active, but can be converted to T3 or rT3)

32
Q

5 monodeiodinase

A

T4 –> T3

33
Q

major target tissues of thyroid hormone

A

liver

kidney

skeletal muscle

34
Q

T4 and T3 enter target cells by ___ then

A

energy-depenent transport

Then most of the T4 is converted to T3 and both T4&T3 are transported to the nucleus to affect transcription.

T3 binds nuclear receptors w greater affinity

35
Q

Physiological effects of thyroid hormone

A
  • Increased basal metabolic rate & general catabolic state
  • Thermogenesis
  • Increased O2 consumption
  • Increases kidney size, renal plasma flow, GFR, and tubule transport
  • Increases cardiac SV, HR, systolic BP
  • Decreased diastolic BP
  • CNS development
  • Increased skeletal growth
  • Regulates reproductive function
36
Q

How does TH increase O2 consumption?

A

Stimualtes Na,K-ATPase, so ADP increases

–> increased mitochondrial O2 uptake

Rate of O2 consumption can be used to classify hypo (150mL/min), euthyroid (250mL/min), and hyperthyroid states (400mL/min)

37
Q

Hyperthyroidism clinical features

A

Tx: B-adrenergic antagonists

38
Q

Thyroid hormone causes increased ____, which shortens diastole, thereby increasing heart rate

A

sarcolemal uptake of Ca2+ shortens idastole

39
Q

Causes of hyperthryoidism

A

Graves: autoimmune; antibody binds TSH receptors and mimics TSH

Neoplasm of thyroid

Thyroid inflammation; Excess TSH, ingestion of excess T3 & T4 ; High iodide

40
Q

Tx of hyper thyroidism

A

Short-term iodine excess

Treatment with thiouracil for 18 months to block TH synthesis

Ablation of thyroid tissues via radioactive iodine or surgery

41
Q

Hypothyroidism

Physiology (42 decks)

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