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