Thyroxine
Chemical
Synthetic levo-isomer of Thyroxine (i.e. synthetic T4)
Use
- Hypothyroidism
- Thyroidectomy
- Pituitary TSH suppression
Presentation
Tablets 25 – 300mcg (varied concentrations)
Dose
Starting dose 25 – 200mcg & titrate with TFTs
Route
PO
Onset
Long t ½
DoA (duration)
Peak effect 4 – 6 weeks
MoA (mechanism)
Liver & kidney convert T4 → T3
T3 alters gene transcription in all cells
PD
- ↑BMR
- ↑Tissue sensitivity catecholamines
- Regulates CHO, protein, fat metabolism
- Facial development & childhood growth
PK
A
OBA 80%
Peak absorption 2hrs
D
Extensive distribution
>99% PPB
M
Deoxidated in liver, kidney, skeletal m.
t ½ 7d
E
Renal elim. metabolism
Adverse Effects
Thyrotoxicosis
Drug interactions i.e. ↓dig levels, drugs with high PPB