What is Synthroid?

Synthroid (levothyroxine) is a synthetic T4 hormone used to treat hypothyroidism, a condition in which the thyroid gland doesn’t produce enough of the hormone.

Synthroid is also used to treat or prevent goiter (enlarged thyroid gland), which can be caused by hormone imbalances, radiation treatment, surgery, or cancer.

Synthroid should not be used to treat obesity or weight problems.

Synthroid

Health Benefits

Synthroid is used to treat hypothyroidism (low thyroid hormone). It is also used to treat or prevent goiter (enlarged thyroid gland), which can be caused by hormone imbalances, radiation treatment, surgery, or cancer.

Synthroid should not be used to treat obesity or weight problems.

Side Effects and Risks

The most common side effects of Synthroid include:

headache,

tremors,

nervousness,

sweating,

changes in appetite,

weight changes,

menstrual changes,

skin rash or dry skin,

hair loss, and

tiredness.

Less common side effects include:

fast or irregular heartbeat,

flushing,

heat intolerance,

fever,

changes in blood pressure,

changes in mood or behavior,

mental/mood changes (e.g., anxiety, depression),

sleep problems (insomnia),

muscle weakness, change in the amount of urine produced, decreased sweating.

Synthroid can also cause serious side effects such as: chest pain, shortness of breath, leg cramps/pain/swelling that goes away when you walk, seizures, or changes in your menstrual periods.

If you experience any of these serious side effects, stop taking Synthroid and contact your doctor immediately.

Interactions and Overdose

Synthroid may interact with other drugs such as: blood thinners, insulin or oral diabetes medications, other thyroid medications, birth control pills or hormone replacement therapy, aspirin or other anti-inflammatory drugs, heart rhythm medications, seizure medications, and digoxin.

Do not take more than the recommended dose of Synthroid without first talking to your doctor. An overdose of Synthroid can be fatal if not treated quickly. Symptoms of a Synthroid overdose include: rapid heartbeat, tremor, shortness of breath, leg cramps, headache, and diarrhea.

Pregnancy and Synthroid

Synthroid is in the FDA pregnancy category A. This means that it is unlikely to cause harm to a developing fetus.

Do not take Synthroid without first talking to your doctor if you are pregnant or could become pregnant during treatment.

It is not known whether Synthroid passes into breast milk. Do not take Synthroid without first talking to your doctor if you are breastfeeding a baby.

Synthroid should not be used in children younger than 6 years of age.

To make sure Synthroid is safe for you, tell your doctor if you have ever had:

a thyroid disorder,

diabetes,

adrenal gland disorder,

pituitary gland disorder,

coronary artery disease, or

if you are taking any other prescription or nonprescription medications.

Dosage

The recommended starting dose of Synthroid for hypothyroidism in adults is 0.075-0.088 mg/day (66-88 mcg/day). The usual maintenance dose is 0.025-0.05 mg/kg (25-50 mcg/kg) per day.

For children, the recommended starting dose of Synthroid is 10-15 mcg/kg per day. The usual maintenance dose is 5-10 mcg/kg per day.

Synthroid should be taken at the same time each day on an empty stomach, 30 minutes to 1 hour before breakfast. It can be taken with or without food, but taking it with food may reduce side effects such as upset stomach, nausea, and diarrhea.

To make sure Synthroid is helping your condition and not causing harmful effects, your blood will need to be tested often. Do not miss any scheduled appointments.

Pharmacokinetics

Synthroid is rapidly absorbed after oral administration.

The mean peak serum T3 and T4 levels occurred 3-5 hours and 6-8 hours, respectively, after a single oral dose of Synthroid in healthy adult subjects.

Serum total T3 and T4 levels increase proportionately with increasing doses of Synthroid from 25 to 300 mcg/day. Accumulation does not occur with once-daily dosing.

In most cases, steady state serum T3 and T4 levels are achieved within 10 days of initiation of treatment and are maintained with once daily dosing. At doses above 200 mcg/day, slight increases in serum T3 and T4 levels may occur.

Serum TSH levels decrease usually within 2 weeks, and are well suppressed at doses of Synthroid above 150 mcg/day. At higher doses (200-300 mcg/day), serum T3 levels may increase slightly while serum T4 levels decrease more appreciably, resulting in a small decrease in the ratio of T3:T4. This change is not thought to be clinically significant.

Mechanism of Action

Synthroid is a synthetic T4 hormone. It works by replacing the missing or deficient amounts of thyroid hormone in the body.