Levothyroxine (T4)

Page Contents

1 WHAT IS IT?
2 WHEN DO WE USE IT?
3 WHEN DO WE AVOID USING IT?
4 HOW IS IT ADMINISTERED?
5 WHAT ARE THE SIDE EFFECTS/TOXICITY?
6 ANYTHING ELSE TO KEEP IN MIND?
7 FURTHER READING

WHAT IS IT?

Levothyroxine (T4) is a synthetic thyroid hormone that is used to replace deficiencies in hypothyroidism.

Thyroid hormone signaling pathway (source)

WHEN DO WE USE IT?

Hypothyroidism, myxedema. Used off label for weight loss (NOT recommended)

WHEN DO WE AVOID USING IT?

Hyperthyroidism, untreated adrenal insufficiency, acute myocardial infarction.

HOW IS IT ADMINISTERED?

Levothyroxine is given orally or IV, and it is absorbed best on an empty stomach (large reduction in absorption if taken with food).

Half life is 7 days, and steady state concentration is achieved in ~ 6 weeks.

Eliminated primarily in the kidney

WHAT ARE THE SIDE EFFECTS/TOXICITY?

Toxicity of this medication is similar to symptoms experienced by hyperthyroidism:

Tachycardia, arrhythmias
Heat intolerance (running “warm”)
Tremors
Weight loss

*Large doses of this medication can be life threatening!

ANYTHING ELSE TO KEEP IN MIND?

T4 might be the preferred thyroid hormone replacement because of its considerably larger half life (compared to T3). This is because T4 will bind albumin with greater affinity.

Levothyroxine is NOT teratogenic! Low levels of T4 can be harmful to the development of the fetus.

Levothyroxine is the L-isomer of thyroxine (administer as a sodium salt)