Thyroid hormones and physiology Flashcards

1
Q

T3 and T4 synthesis

A
  1. Iodine + Tyrosine
    - Forms monoiodotyrosine [MIT] or di-iodotyrosine [DIT]
  2. MIT+DIT= Triiodothyronine [T3]
    or

DIT+DIT= Tetraiodothyronine/ Thyroxine [T4]

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Source of iodine

A

Plants grown in rich iodine environment.

Meat which were fed plants grown in rich iodine environment.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Iodothyronine deiodinase

A

Enzyme that converts T4 into T3 or rT3 in target tissues.

When thyroid hormones are synthesised in the thyroid, 95% are released as T4.

T3 is more biologically active than T4 [40x more]
- 80% of conversion

rT3 is biologically inactive
- 20% of conversion

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Halogenase

A

Enzyme that degrades MIT and DIT

- Releases free iodide ions.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Thyroglobulin

A

Protein that combines with iodide released from MIT and DIT breakdown.

  • Combination forms precursor of thyroid hormones T3 and T4.
  • Source of iodine that combines with tyrosine.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

T4

  • Name
  • Synthesis
  • Action
  • Half life
A

Tetraiodothyronine/ Thyroxine

Formed from 2 DIT molecules.

Main hormone produced and secreted by the thyroid.

Converted into T3 in target tissues using iodothyronine deiodinase.

Half life= 6-8 days.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

T3

  • Name
  • Synthesis
  • Half life
A

Triiodothyronine

Formed from MIT+DIT

Made in thyroid [5% of secreted thyroid hormones].

Converted from T4 in target tissue.
- Using iodothyronine deiodinase.

More biologically active that T4.

Half life= 1 day

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Iron and thyroid activity

A

Iron concentration has an effect on thyroid activity
- Iron improves thyroid hormone indices

Iron deficiency can see a decrease in T4 and T3—> Hypothyroidism.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Role of radioactive iodine on thyroid function.

A

Radioactive iodine damages thyroid glands and can trigger the formation of neoplasms.
- Chronic exposure to digoxins= decrease in thyroid function.

Example
- Chernobyl incident = high incidence of thyroid cancer decades later.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Control of thyroid hormone secretion.

A

Hypothalamus secretes TRH
- Triggers anterior pituitary to release TSH.

TSH travels in systemic circulation and triggers release of Thyroid hormones from the thyroid.

Feedback regulation
- T3 and T4 negatively inhibit the release of TSH and TRH.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Transport of thyroid hormones.

A

Thyroid hormones are not water soluble.
- Mainly bound to plasma protein [99%]

T4

  • TBG: Thyronine binding globulin [75%]
  • TBPA: Thyroxine binding prealbumin [15-20%]
  • Albumin [5-10%]
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Overall biological effects of thyroid hormones

A

Increase basal metabolic rate by increasing the size and number of mitochondria.

Also important in the normal development and growth of prepubescent children.
- Important in the development of the CNS[ myelination of nerve fibres]

Increases activity of metabolically important enzymes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Metabolic effects of thyroid hormones [3]

A

Increases lipid turnover
- Increased rate of synthesis, mobilisation and degradation.

Increased protein synthesis.

Increased carbohydrate metabolism.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Thyroid hormones on carbohydrate metabolism. [4]

A

Increases glycogenesis.

Increases glucose uptake in muscles and adipose tissue.

Potentiates the effects of insulin and catecholamines.

Increases glucose absorption in the gastrointestinal tract.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Organs thyroid hormones ineffective on [6]

A

Thyroid

Anterior pituitary

Brain

Testes, Uterus

Spleen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Levothyroxine

  • Drug type
  • Indication
  • Route of administration
  • Dose
A

Manufactured thyroxine [T4]

Indication

  • Hypothyroidism
  • Can treat goitre by inhibiting TSH.

Administration

  • Oral
  • IV

Dose

  • 50-100 micrograms/day
  • 25 micrograms/daily, titrated in older people [50+]
17
Q

Levothyroxine

  • Oral bioavailability
  • Protein binding
  • Metabolism
  • Excretion
  • Half life
A

Oral bioavailability= 100%
- Orally inactive

Protein binding= 99%
- Not soluble in water.

Metabolism
- Liver [glucorodination]

Half life
- 7 days

Excretion
- Urine

18
Q

Levothyroxine

- Adverse effects

A

Similar to symptoms of hyperthyroidism

  • Insomnia
  • Weight loss
  • Tremors
  • Palpitations, arrhythmias
  • Diarrhoea
19
Q

Carbimazole

  • Drug type
  • Mechanism
  • Indications
  • Dose
  • Administration route
A

Anti-thyroid pro-drug

  • Inhibits the incorporation of iodide into thyroglobulin= inhibits the production of T3 and T4
  • Inhibits peroxidase enzyme

Indications
- Hyperthyroidism

Dose
- 5-15mg/day

Administration
- Oral

20
Q

Carbimazole

  • Oral availability
  • Protein binding
  • Metabolism
  • Half life
  • Excretion
A

Oral availability = 90%

Protein binding =85%

Metabolism
- After absorption, converted into methimazole [active form]

Half life
- 6.4 hrs

Excretion
- Urine

21
Q

Carbimazole

- Adverse effects

A

Rashes and pruritus

Neutropenia
Agranulocytosis

Teratogenicity

22
Q

Propylthiouracil

  • Drug type
  • Mechanism
  • Indications
  • Dose
A

Anti-thyroid drug

  • Inhibits thyroperoxidase and tetroiodothyronine deiodinase
  • Prevents iodide + thyroglobulin incorporation
  • Inhibits the conversion of T4 to T3.

Indications

  • Hyperthyroidism [inc. Graves’ disease]
  • Hyperthyroidism in pregnancy

Dose
- 50-150mg/day

23
Q

Propylthiouracil

  • Oral bioavailability
  • Protein binding
  • Metabolism
  • Half life
  • Excretion
A

Oral bioavailability =80-95%

Protein binding= 70%

Metabolism
- Hepatic

Half life
- 2 hrs [good for breastfeeding]

Excretion
- Renal [urine]

24
Q

Propylthiouracil

- Adverse effects

A

Similar to carbimazole + liver problems:

  • Pruritus
  • Rashes
  • Agranulocytosis
  • Liver injury
25
Q

Potassium perchlorate

as a drug

A

Anti-thyroid drug

- Competes with iodide for the iodide uptake pump

26
Q

Radioactive iodine

A

Used to treat hyperthyroidism

Inhibits T3+T4 secretion by suppressing the thyroid

27
Q

Jod-Basedow phenomenon

A

Hyperthyroidism triggered by excess iodine consumption.

Iodine can be consumed via diet [vitamins, cough remedies] or via injections.

28
Q

Lithium on the thyroid

A

Lithium can trigger hypothyroidism.

Induces goitre
- Decreases T4 production