6. The synthesis of thyroid hormone
and action and effects of
antithyroid agents
7. Thyroid hormone
synthesis and action
Effects of antithyroid
agents
1 Active accumulation of
iodide into the gland
Basis for selective cell
destruction of 131I
2 Oxidation of iodide by
peroxidases
Inhibited by thioamides
3 Iodination of tyrosyl
residues on thyroglobulin
from MIT and DIT
Inhibited by thioamides
4 Coupling of MIT and DIT
to form T3 and T4
Inhibited by thioamides
5 Proteolytic release of T3
and T4 from thyroglobulin
Inhibited by high doses of
iodide
6 Conversion of T4 to T3 via
5’ deiodinase in
peripheral tissues
Inhibited by propranolol,
and propylthioiracil
8.
9. Thioamides: Propylthiouracil and Methimazole
• Use in uncomplicated hyperthyroid conditions;
• High-dose propylthiouracil inhibits 5' deiodinase
• Common maculopapular rash
• Less common prothrombin, hypersensitivity,
and immune-based arthralgia,
• jaundice, lupus, and vasculitis
• Both drugs cross the placental barrier, but PTU
is safer in pregnancy because it is extensively
protein bound
14. Iodide
• KI+ iodine (Lugol's solution) possible use in
thyrotoxicosis: used preoperatively, →↓
gland size, fragility, and vascularity
• No long-term use because thyroid gland
"escapes" from effects after 10-14 days