4. participates in normalizing growth and development
and energy levels and the proper functioning and
maintenance of tissues / organs
critical for the nervous, skeletal and reproductive
tissues
it affects secretion and degradation rates of all
hormones
5. Secretion of the following hormones:
◦ Triiodothyronine (T3) ; 59% iodine
◦ Tetraiodothyronine (T4; also known as
thyroxine); 65% iodine
◦ Calcitonin
6. Iodide Metabolism
◦ The recommended daily adult iodide (I-) intake is 150 mcg
Biosynthesis of Thyroid Hormones
Transport of Thyroid Hormones
◦ thyroxine-binding globulin (TBG)
◦ about 0.04% of total T4 and 0.4% of T3 exist in the free
form.
7.
8.
9. Iodide trapping
Oxidation of iodide to iodine
Iodide Organification
Formation of T4 and T3
Release of T4 and T3
10. PERIPHERAL METABOLISM OF THYROID
HORMONES
The primary pathway for the peripheral metabolism of thyroxine (T4) is
deiodination deiodination of T4 may occur by monodeiodination of the outer
ring, producing 3,5,3'-triiodothyronine (T3), which is three to four times more
potent than T4
11.
12. A model of thyroid hormone action is depicted in Figure 38-4
Figure 38-4. Regulation of transcription by thyroid hormones
•T3 and T4 are
triiodothyronine
and thyroxine,
respectively.
•PB, plasma
binding protein;
•F, transcription
factor; R, receptor;
PP, proteins that
bind at the
proximal promoter.
BASIC PHARMACOLOGY OF THYROID &
ANTITHYROID DRUGS
THYROID HORMONES
13. A syndrome resulting from a deficiency of thyroid hormones
and is manifested largely by a reversible slowing down of all
body functions.
There is a striking retardation of growth and development.
In children, manifested as dwarfism and severe MR.
14. synthetic levothyroxine (synthetic T4)
Brand names: Eltroxin , Euthyrox,Levoxyl, Levothroid, Synthroid
for hormone replacement therapy in hypothyroidism
DOSE
Infants and Children require more T4/Kg body weight than adults
Average dose for an infant -10-15 micrograms/kg/d
Average dose for an adult – 1.7micrograms/kg/d
Once daily
Pharmacokinetics
should be taken 30min before or 1 hour after meals (delayed absorption for
soy, other foods and drugs)
takes 6-8 weeks to reach steady state levels
Labs should be repeated after 2 months
15. Reasons for its use:
◦ stability
◦ content uniformity
◦ low cost
◦ lack of allergenic foreign
protein
◦ easy laboratory
measurements of serum
levels
◦ long half-life (7days)
◦ once a day dosing
16. Uses
Hormone replacement therapy
In young patients or those with mild disease- full replacement therapy started
In older patients and in patients with cardiac disease -start treatment with
reduced dosage
Myxedema Coma – medical emergency
Loading dose – of T4 – 300-400micrograms I/V initially f/by `50micrograms
daily
I/V T3 – more cardiotoxic and difficult to moniter
Hypothyroidism and Pregnancy – daily dose –adequate
17. synthetic liothyronine
(synthetic T3) is 3-4x
more potent
(Cytomel,Triostat)
not used alone for long
term treatment secondary
to short half life and large
peaks in serum T3 levels
increase risk for cardiac
side effects secondary to
hyperthyroid states during
treatment
18. A thyroid disorder caused by an antibody-mediated
auto-immune reaction, but the trigger for this reaction is
still unknown.
27. Thioamides should be given initially and stop 5-7 days before
radioactive iodine administration
131I dosage generally ranges between 80-120uCi/g of
estimated thyroid wt. corrected for uptake. May be repeated
after 6 months
Adverse effects
◦ permanent hypothyroidism
◦ potential for genetic damage
◦ may precipitate thyroid crisis
28. Monovalent anions such as perchlorates, pertechnetate and
thiocyanate can block uptake of iodide by the gland by
competitive inhibition
can be overcome by large doses of iodides
useful for iodide-induced hyperthyroidism (amiodarone-
induced hyperthyroidism)
rarely used due to its association with aplastic anemia
29. major anti-thyroids before the
introduction of thioamides
(1950s)
preparations:
◦ strong iodine solution (Lugol’s)
◦ potassium iodide
◦ iodone
30. MOA:
◦ acutely blocks release of thyroid hormone from the gland by
inhibiting thyroglobulin proteolysis
inhibit iodide organification
Uses:
◦ useful in thyroid storms: 2-7 days
◦ Preoperatively - iodides decrease vascularity, size and fragility of
hyperplastic gland
Caution:
◦ it may delay onset of thioamide effects; should be given after
initiation of thioamides
◦ The gland will escape from inhibition after 2-8 weeks.
31. Iodinated contrast media
Ipodate (oral)
Iopanoic acid (oral)
Diatrizoate (intravenous)
valuable in hyperthyroidism (but is not labeled for this
indication)
MOA: inhibits conversion of T4 to T3 in the liver, kidney,
brain and pituitary
Another MOA is due to inhibition of hormone release
secondary to iodide levels in blood
Useful in thyroid storms (adjunctive therapy)
32. Drugs: Propranolol, Metoprolol, Atenolol
MOA:
◦ Membrane-stabilizing action: inhibits T4 to T3
◦ Ameliorate many disturbing s/sxs of hyperthyroidism
secondary to increased circulating catecholamines by blocking
beta receptors
Indications: Grave’s, Thyroid storm
33. Prednisone is given for patients with Grave’s
ophthalmopathy
1mg/kg/day (60mg/day 3 divided doses); if it should be
given for more than 4 weeks, taper to decrease risk of
adrenal crisis
34. Sudden exacerbation of throtoxic symptoms
Life threatening condition
Vigorous management
◦ Propanalol 1-2mg i/v or 40-80mg PO Q6h
◦ Diltiazem 90-120mg Po Q8-6 hrs or 5-10mgs intravenous
infusion/hour