thyroid drugs

1. Thyroid gland
• The normal circulating thyroid hormones are
Thyroxine T4 (90%),Triiodothyronine T3 (9%)
and rT3 (1%).
• Reverse T3 (rT3) is biologically inactive form of
T3.
• Thyrotoxicosis or hyperthyroidism (Graves
disease) is the clinical syndrome caused by an
excess of thyroid hormones.
• Hypothyroidism (Hashimoto's thyroiditis) is a
clinical disorder in which there is a deficiency of
thyroid hormones.

2. THYROID
HORMONES
Increases basal
metabolic rate.
Potentiates brain
development.
Potentiates the
beta effects of
catecholamines.

3. Thyroid gland
Metabolism of thyroid hormones :
• The primary metabolism of thyroxine is
deiodination.
• Deiodination of T4 may occur by
monodeiodination of outer ring producing 3,5,3’-
triiodothyronine - T3 ( T3 is 4 times more potent
than T4) by 5’-monodeiodinase.
• Deiodination of inner ring produce 3,3’,5’ –
reverse triiodothyronine – rT3 (inactive) by 5-
monodeiodinase.

4. Outer ring Inner ring
5’-monodeiodinase. 5-monodeiodinase.

5. Thyroid hormones
Metabolism of Thyroid hormones :
• Drugs like beta blockers, high dose
propylthiouracil and steroid inhibit
the 5’-deiodinase activity necessary for
conversion of T4 to T3 resulting in low
T3 and high of rT3.

6. Thyroid gland
Mechanism of action of thyroid
hormones :
• T4 dissociate from thyroxine binding
globulin in plasma before entry into cells.
• In the cells, T4 is enzymatically
deiodinated to T3 which enters nucleus
and attaches to specific receptors which
promotes protein synthesis.

7. Anti-thyroid drugs

8. Anti-thyroid drugs

9. Anti-thyroid drugs
Drugs used for the treatment of
hyperthyroidism :
Inhibition of hormone synthesis :
•Propylthiouracil and Methimazole.
Blockade of hormone release :
•Iodides, Iodinated contrast media.
Radioactive Iodine 131
Anion Inhibitors :
• Perchlorates, Thiocynates.
Beta blocking drugs : Propranolol.

10. Anti-thyroid drugs
peroxidase

11. Anti-thyroid agents
THIOAMIDES :
• Inhibit hormone synthesis by inhibiting
peroxidase.
• Propylthiouracil also inhibits peripheral
deiodination of T4 and T3.
• Methimazole is more potent and longer
acting than propylthiouracil.
• Slow in onset ~ 4 weeks.

12. Anti-thyroid drugs
THIOAMIDES :
• These are used for treatment of mild
thyrotoxicosis and in preparation of
surgery.
• Propylthiouracil is relatively safe and
preferred in pregnancy.

14. Anti-thyroid drugs
THIOAMIDES : ADVERSE
EFFECTS
• Common adverse effects includes
maculopapular rash, arthralgia and
vasculitis.
• Agranulocytosis – reversible.

15. Anti-thyroid drugs
Potassium iodide : Inorganic
iodide
• It blocks the organification and release
through inhibition of proteolysis – known
as Wolff – Chaikoff effect.
• It decrease the size and vascularity –
used before surgery.
• Rapid improvement in 2-5 days.
• It is an ideal agent for the treatment of
severe thyrotoxicosis and preoperatively.

16. Anti-thyroid drugs
Potassium iodide : Inorganic iodide
• Anti-thyroid effect is not for long term as
gland ‘escapes’ from its effect.
• This is the most effective way of limiting
the potential damage to thyroid gland
by radiation emergencies.

17. Anti-thyroid drugs
Potassium iodide : Inorganic iodide
• Chronic use in pregnancy avoided –
fetal goiter.
• Adverse reactions to iodine includes –
iodine induced thyrotoxicosis (Jod
Basedow’s phenomenon) at low doses.
• Others includes – salivary gland
inflammation and acne.

18. Anti-thyroid drugs
Radioactive Iodine :
• I-131 is the only isotope used in treatment of
thyrotoxicosis while others are used in
diagnosis.
• Administered as sodium I–131 orally.
• Therapeutic effect depends on emission of
beta rays – destroys the thyroid gland.
• Patients eventually becomes hypothyroid –
managed with thyroxine.

19. Anti-thyroid drugs
Radioactive Iodine :
• I–131 serves as alternative to surgery.
• Easy, effective, low cost and absence
of pain are the advantages.
• Not advisable for pregnant women.

20. Anti-thyroid drugs
ANION INHIBITORS :
• Monovalent ions like perchlorate,
pertechnetate, thiocyanate can
competitively block the uptake of iodine.
• Anion inhibitors are uncommon in use
because of aplastic anemia.
• These are effective in iodine induced
hyperthyroidism

21. Anti-thyroid drugs
Iodinated contrast media :
Diatrizoate / Iohexol :
• They are valuable in hyperthyroidism
and as adjunctive in thyroid storm.
• They inhibit the peripheral conversion of
T4 into T3.
• Inhibition of hormone release is an
additional mechanism.

22. Anti-thyroid drugs
Other Anti-thyroid drugs :
• Propranolol is used in the management
of cardiac symptoms of thyrotoxicosis.
• Lithium is known to inhibit synthesis and
release of thyroid hormones.
• Amiodarone can also result in
hypothyroidism.

23. Thyroid gland
• The extreme manifestations of
untreated hypothyroidism is
myxedema coma with a mortality ~
50%.
• Myxedema coma is treated with
intravenous T4 or sometimes T3.
• Levothyroxine T4 is the best choice for
replacement therapy.
• Thyroid hormones replacement is
monitored by plasma TSH.

24. Thyroid gland
Thyroid storm: Thyrotoxic crisis, is an
acute, life-threatening state induced by
excessive release of thyroid hormones.
• Propranolol to minimize cardiac symptoms
• High-dose propylthiouracil is preferred
because of its ability to inhibit peripheral
conversion of T4 to T3
• Potassium iodide used to block the
release of thyroid hormones