Synthroid Indication:

For use alone or in combination with antithyroid agents to treat hypothyroidism, goiter, chronic lymphocytic thyroiditis, myxedema coma, and stupor.

Synthroid Mechanism Of Action:

Synthroid acts like the endogenous thyroid hormone thyroxine (T4, a tetra-iodinated tyrosine derivative). In the liver and kidney, T4 is converted to T3, the active metabolite. In order to increase solubility, the thyroid hormones attach to thyroid hormone binding proteins, thyroxin-binding globulin, and thyroxin-binding prealbumin (transthyretin). Transport and binding to thyroid hormone receptors in the cytoplasm and nucleus then takes place. Thus by acting as a replacement for natural thyroxine, symptoms of thyroxine deficiency are relieved.

Synthroid Drug Interactions:

Anisindione Thyroid hormones increase the anticoagulant effect
Acenocoumarol Thyroid hormones increase the anticoagulant effect
Dicumarol Thyroid hormones increase the anticoagulant effect
Warfarin Thyroid hormones increase the anticoagulant effect
Calcium Calcium decreases absorption of levothyroxine
Iron Iron decreases absorption of levothyroxine
Colestipol The resin decreases the absorption of thyroid hormones
Cholestyramine The resin decreases the absorption of thyroid hormones
Digoxin The thyroid hormone decreases the effect of digoxin
Raloxifene Raloxifene decreases absorption of levothyroxine
Sucralfate Sucralfate decreases the effect of levothyroxine

Synthroid Food Interactions:

No iron within 4 hours of taking this medication.
Take 30-60 minutes before breakfast.
Consistent administration in relation to meals is recommended.

Synthroid Generic Name:


  • (-)-Thyroxine
  • 3,3',5,5'-Tetraiodo-L-thyronine
  • L-Thryoxin
  • L-Thyroxine
  • Laevothyroxinum
  • Levo-t
  • Levothyroxine Sodium
  • Tetraiodothyronine
  • THX
  • Thyroxine iodine
  • Levothyroxin
  • T4 levothyroxine
  • O-(4-Hydroxy-3,5-diidophenyl)-3,5-diiodo-L-tyrosine

Drug Type:

Small Molecule; Approved


Bioavailability varies from 48% to 80%. Human studies have confirmed the importance of an intact jejunum and ileum for levothyroxine absorption and have shown some absorption from the duodenum.

Toxicity (Overdose):

LD50=20 mg/kg (orally in rat). Hypermetabolic state indistinguishable from thyrotoxicosis of endogenous origin. Symptoms of thyrotoxicosis include weight loss, increased appetite, palpitations, nervousness, diarrhea, abdominal cramps, sweating, tachycardia, increased pulse and blood pressure, cardiac arrhythmias, tremors, insomnia, heat intolerance, fever, and menstrual irregularities.

Protein Binding:

> 99%


Hepatic (primarily via cytochrome P450 isozyme 3A4).

Half Life:

6 to 7 days

Dosage Forms of Synthroid:

Powder, for solution Intravenous
Tablet Oral

Chemical IUPAC Name:

(2S)-2-amino-3-[4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodophenyl]propanoic acid

Organisms Affected:

Humans and other mammals

Synthroid to general, pharmacology

General, pharmacology..