Reviewed on 8/6/2021

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:

  • increased cholesterol levels
  • depression
  • fatigue
  • hair loss memory loss
  • dry, rough skin, and
  • constipation

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.


Where is the thyroid gland located? See Answer

What Are the Dosages of Levothyroxine?

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

What Are Side Effects Associated with Using Levothyroxine?

Common side effects associated with using levothyroxine include:

  • Chest pain (angina pectoris)
  • Joint pain
  • Congestive heart failure
  • Flushing
  • Increased pulse
  • Heart attack
  • Palpitations
  • Irregular heartbeat
  • Cramps
  • Diarrhea
  • Anxiety
  • Choking sensation
  • Mood changes
  • Headache
  • Heat intolerance
  • Insomnia
  • Muscle weakness
  • Pseudomotor cerebri
  • Hair loss
  • Weight loss
  • Infertility
  • Shortness of breath
  • Fever
  • Fast heart rate
  • Tremor
  • Decreased bone mineral density

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

What Other Drugs Interact with Levothyroxine?

  • Levothyroxine has moderate interactions with at least 34 different drugs.
  • Levothyroxine has mild interactions with at least 34 different drugs.

This document does not contain all possible interactions. Therefore, before using this product, tell your doctor or pharmacist of all the products you use. Keep a list of all your medications with you, and share the list with your doctor and pharmacist. Check with your physician if you have health questions or concerns.


Hyperthyroidism Symptoms and Treatment See Slideshow

What Are Warnings and Precautions for Levothyroxine?


Thyroid hormones, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss.

In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction; larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects.

This medication contains levothyroxine. Do not take Synthroid, Levoxyl, L Thyroxine, Levo T, Levothroid, Levothyroxine T4, Levoxine, Tirosint, or Unithroid if you are allergic to levothyroxine or any ingredients contained in this drug.

Keep out of reach of children. In case of overdose, get medical help or contact a Poison Control Center immediately.


Hypersensitivity to thyroid hormone or other ingredients

Acute MI, thyrotoxicosis, untreated adrenal insufficiency

Synthetic thyroid hormone should not be used in the treatment of obesity or infertility

No contraindications reported by manufacturer for myxedema coma when treated with injection

Effects of Drug Abuse

There are no known effects of drug abuse of thyroid hormone in synthetic form.

Short-Term Effects


Long-Term Effects



Avoid under-treatment or over-treatment, which may result in adverse effects.

Use caution in cardiovascular disease, HTN, endocrine disorders, osteoporosis, or myxedema.

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

Do NOT generally use levothyroxine sodium preparations interchangeably, due to narrow therapeutic index.

Check for bioequivalence if switching brands/generics.

Avoid beta-blocker use when patients are being treated for cardiovascular complications caused by Levothyroxine abuse.

Synthroid and Unithroid tablets contain lactose.

Not recommended for TSH suppression in patients with thyroid nodules.

Levoxyl tablets swell in mouth: take with full glass of water to avoid choking.

Avoid use in postmenopausal women >60 years with osteoporosis, cardiovascular disease, or systemic illness.

Avoid use in patients with large thyroid nodules or long-standing goiters, or low-normal TSH levels.

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

Use caution in patients with adrenal insufficiency; may exacerbate symptoms or aggravate them; treatment with glucocorticoids should precede levothyroxine therapy if adrenal insufficiency present.

Pregnancy and Lactation

Levothyroxine is generally acceptable to use during pregnancy. Controlled studies in pregnant women show no evidence of fetal risk. Levothyroxine enters breast milk; use caution while breastfeeding.

Medscape. Synthroid. Levothyroxine.

Health Solutions From Our Sponsors