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Flashcards in Thyroid Hormones Deck (38):
1

triiodothyronine

T3

2

thyroxine

T4

3

Thyroid hormone preparation of T4

Levothyroxine (given orally, long half life)

4

T3 aka

triiodothyronine

5

T4 aka

thyroxine

6

Thyroid hormone preparation of T3

Liothyronine (more potent, faster acting but short half life, injectable)

7

When do you give T3?

T3 given only when an immediate response is needed (E.g. in myxodema coma)

8

MOA of thyroid hormones

➢ Thyroid hormones are attached to plasma carrier proteins.
• Hormones dissociate from carrier proteins enter the cell by active transport

➢ Within the cell T4 converted to T3 – enters nucleus- bind to thyroid hormone receptor - localized in the nucleus --> Stimulates transcription of particular genes

9

Levothyroxine

T4 hormone prep

10

Effects of Thyroid hormones on growth & development

• Essential for normal physical & mental growth, hence cretinism - mental retardation

11

caloric effects of thyroid hormones

• ↑ BMR and
• ↑ manitain body temperature

12

Liothyronine

T3 hormone prep

13

cardiovascular effects of thyroid hormones

• ↑ HR and
• ↑ peripheral resistance

14

metabolic effects of thyroid hormones

• ↑ blood sugar,
• ↑ synthesis of fatty acids and
• decrease plasma CH and TG levels

15

Use of thyroid hormones

➢ Hypothyroidism caused by:

• Hashimoto’s disease
• Myxedema
• Following surgical ablation of thyroid gland

16

hypothyroidism signs and symptoms

• anemia;
• hyponatremia,
• galactorrhea,
• goiter;
• hard, pitting edema of the lower extremities
• delayed return of deep tendon reflexes

• Any combination of a few or many signs and symptoms may occur at any given time.
• Some signs and symptoms, such as galactorrhea, anemia, and skin changes are seen in individuals in later stages of hypothyroidism.

17

Thyroid Inhibitors:

• Perchlorate,
• thiocyanate,
• goitrin

18

Thyroid Hormone Synthesis

1. Uptake of Iodide (iodide trapping)

2. Iodide organification
• (oxidation and iodination)

3. Coupling of MIT and DIT

4. Secretion of thyroid hormones

5. Conversion of T4 to T3

19

Ionic Inhibitors

Affects uptake of Iodine

• Perchlorate (ClO4- )
• thiocyanate (SCN- )

➢ No Therapeutic use: only Diagnostic

20

Monovalent anions

Affects uptake of Iodine

• Resemble Iodide
• Inhibit transport of iodide into thyroid

➢ No Therapeutic use: only Diagnostic

21

Thyroid peroxidase

Oxidizes iodide to iodine

22

Thioamides

Antithyroid drug!

➢ Iodine rapidly iodinates tyrosine residues in thyroglobulin forming MIT (mono-iodotyrosine) and DIT (di-iodotyrosine)

• Inhibited by: Thioamides

23

Coupling of MIT and DIT

➢ Coupling reaction involves oxidation by:
• Thyroid peroxidase

1. MIT + DIT --> T3
2. DIT + DIT --> T4

➢ Stored in thyroglobulin in colloid matrix
• Inhibited by thioamides -

examples:
• Methimazole
• Propylthiouracil

24

Thioamides MOA

➢ Inhibit thyroid peroxidase :
• Block iodine oxidation
• Coupling of MIT and DIT

25

Propylthiouracil MOA

Inhibit peripheral conversion of T4 to T3

• Block synthesis T3 and T4 not release, therefore slow onset of action

26

Thioamides A/E

➢ Relatively low incidence (3 – 12%)
• Maculopapular pruritic rash
• Hypothyroidism
• Gastrointestinal intolerance
• Agranulocytosis (

27

Secretion of T4 and T3 Inhibited by:

Iodine and Iodides

28

Secretion of T4 and T3

➢ Proteolysis - release of T4 and T3 stored within thyroglobulin (TG)
• MIT and DIT deiodinated; iodine reutilized
➢ Inhibited by Iodine and Iodides

29

Iodides

➢ Inhibit TH release - thyroid constipation
➢ Decreased size & vascularity of gland
➢ Improvement of thyrotoxic symptoms occurs rapidly – used in thyroid storm

30

Conversion of T4 to T3 Inhibited by

• Propranolol
• Propylthiouracil
• Glucocorticoids (prednisolone)

31

Conversion of T4 to T3:

➢ 80% of T4 converted to T3 in peripheral tissues

• De-iodination of T4 (thyroxine) to active T3 (triiodothyronine)

32

Lugol’s Iodine and Na/K Iodide:
• use
• toxicity

Use:
• Preoperatively- thyroidectomy
• Thyroid storm

Toxicity:
• Iodism- rash, rhinorrhoea, ulcers, conjuctivitis

33

Radioactive Iodine

• what
• half life

I-131

➢ Rapidly absorbed (orally)
➢ Concentrated in thyroid
➢ Emits b radiation --> destruction of thyroid gland

• half life (5 days)

34

Use of radioactive iodine
• advantages
• disadvantages
• contraindicated

Use: Hyperthyroidism

Advantage:
• Inexpensive, simple, outpatient basis
• No surgical risk
• Permanent cure

Disadvantage:
• response slow
• Hypothyroidism

Contraindicated:
• pregnancy,
• young patients

35

Treatment of Hyperthyroidism

➢ Antithyroid drug therapy
• Propylthiouracil, methimazole
➢ Thyroidectomy
➢ 131Iodine

36

Thyroid Storm
• triggers
• treatment
• what is it

aka thyrotoxic crisis
• high mortality

Triggers:
• Acute illness, surgery, radioiodine in an untreated thyroid.

Treatment:
• Large doses of Propylthiouracil: inhibit the peripheral conversion
• Propranolol - control cardiac manifestations
• Potassium iodide – inhibit release of thyroid hormones
• Glucocorticoids- protect against shock, block conversion of T4 to T3

37

What is Thyroid Storm

Thyroid storm is a crisis or life-threatening condition characterized by an exaggeration of the usual physiologic response seen in hyperthyroidism.

Whereas hyperthyroidism can cause symptoms such as sweating, feeling hot, palpitations and weight loss - symptoms of thyroid storm are more severe, resulting in complications such as:

• fever
• dehydration
• rapid heart rate
• nausea/vomiting
• diarrhea
• irregular heart beat
• weakness
• heart failure
• confusion/disorientation
• death

38

Propranolol

beta blocker

➢ Many symptoms of thyrotoxicosis - sympathetic stimulation- palpitation, nervousness, tremors

• b-blockers - Block physiological effects of sympathetic nervous system stimulation