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Brand Name: Synthroid, Levoxyl, L Thyroxine, Levo T, Levothroid, Levothyroxine T4, Levoxine, Tirosint, and Unithroid

Generic Name: levothyroxine sodium

Drug Class: Thyroid products

What Is Levothyroxine and How Does It Work?

Levothyroxine sodium is a drug approved by the FDA as an oral and injectable prescription thyroid hormone medication that is used to treat underactive thyroid (hypothyroidism) and other conditions. Hypothyroidism is a medical term that refers to any state in which a person's thyroid hormone production is below normal. This medication replaces or provides thyroid hormone support, which is normally produced by the thyroid gland. Low thyroid hormone levels can occur naturally or when the thyroid gland is injured by radiation/medications or removed by surgery. Having enough thyroid hormone is important for maintaining normal mental health and physical activity. In children, having enough thyroid hormone is important for normal mental and physical development. Levothyroxine is also know as L-thyroxine.

Symptoms of Hypothyroidism

The symptoms of hypothyroidism are depend on the health of the thyroid and the efficient production of the thyroid hormone, but can include:

Levothyroxine sodium is also used to treat other types of thyroid hormone and thyroid disorders and conditions (such as certain types of goiters, thyroid cancer).

Levothyroxine sodium should not be used to treat infertility unless it is caused by low thyroid hormone levels.

Levothyroxine sodium is available under the following different brand names: Synthroid, Levoxyl, L Thyroxine, Levo T, Levothroid, Levothyroxine T4, Levoxine, Tirosint, and Unithroid.

Dosages of Levothyroxine Should Be Given As Follows:

Adult and Pediatric Dosage Forms & Strengths

Tablet form of medication

  • 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg
  • 25 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg

Capsule (Tirosint) form of medication

  • 13 mcg, 25 mcg, 50 mcg, 75 mcg, 88 mcg
  • 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg

Powder for injection form of medication

  • 100 mcg/vial
  • 200 mcg/vial
  • 500 mcg/vial

Dosage Considerations

Mild Hypothyroidism

1.7 mcg/kg or 100-125 mcg PO qDay; not to exceed 300 mcg/day

>50 years (or <50 yr with CV disease)

  • Usual initial drug dose: 25-50 mcg/day
  • May adjust drug dose by 12.5-25 mcg q6-8Week

>50 yr with CV disease

  • Usual initial drug dose: 12.5-25 mcg PO qDay
  • May adjust drug dose by 12.5-25 mcg q4-6weeks until patient becomes euthyroid and serum TSH concentration normalized; adjustments q6-8weeks also used
  • Dose range: 100-125 mcg PO qDay

Severe Hypothyroidism

Initial drug dose: 12.5-25 mcg PO qDay

Adjust drug dose by 25 mcg/day q2-4Week PRN

Subclinical Hypothyroidism

Initial: 1 mcg/kg PO qDay may be adequate, OR

If replacement therapy not initiated, monitor patient annually for clinical status

Myxedema Coma

300-500 mcg IV once, THEN 50-100 mcg qDay until patient is able to tolerate oral administration; may consider smaller doses in patients with cardiovascular disease

Dosing Considerations

Avoid under-treatment or over-treatment with this drug. This may result in adverse effects.

Initiate lower dose of medicine in elderly, those with angina pectoris, cardiovascular disease, or in those with severe hypothyroidism.

Long-term therapy decreases bone mineral density; use lowest dose in postmenopausal women and women using suppressive doses.

Lower dose of Tirosint capsules may be required compared with standard T4 tablets for hypothyroidism in patients with impaired gastric acid secretion to reach their target TSH levels; Tirosint has shown improved absorption compared with conventional T4 tablets

Check for bioequivalence if switching brands/generics, OR every week after switching from one levothyroxine sodium preparation to another.

Monitor serum thyroid levels; patient may be asymptomatic


  • Initially evaluate patients every 6-8 weeks
  • Once normalization of thyroid function and serum TSH (thyroid-stimulating hormone) concentration achieved, evaluate every six to 12 months.


Take tablets with full glass of water preferably 30 min to 1 hr before breakfast on empty stomach

Do not use foods that decrease absorption (soybean products) for administering levothyroxine

Administer oral levothyroxine >4 hr apart from drugs known to interfere with absorption

IV/IM 50% of PO

Patients unable to swallow intact tablets

  • Crush appropriate dose and place in 5-10 mL of water
  • Administer resultant suspension by spoon or dropper immediately, do NOT store
Medically Reviewed by a Doctor on 4/14/2017

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