Synthroid dosage, forms, and strengths

Synthroid (levothyroxine sodium) is a brand-name prescription drug. It is a synthetic version of the thyroid hormone (thyroxine, T4) in the body. The thyroid hormone binds to the thyroid gland which is involved in metabolism, growth, and development. Synthroid is FDA-approved to treat hypothyroidism. It is also used to suppress thyroid-stimulating hormone (TSH) in patients with thyroid cancer.

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Synthroid forms and strengths

Synthroid is typically taken as a tablet by mouth once daily, on an empty stomach.

• Tablets (in microgram): 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg

Synthroid dosage for adults

Synthroid dosing is dependent on the patient’s weight and medical condition.

Synthroid dosage for hypothyroidism

Hypothyroidism affects the endocrine system and thyroid function. It causes the body to not have enough of certain thyroid hormones. It can be congenital (from birth) or caused by thyroid gland removal or ablation, Hashimoto’s thyroiditis, pituitary gland failure, iodine deficiency, or even certain medications. This hypothyroid state (low thyroid hormone level) causes symptoms of weight gain, fatigue, goiter (enlarged thyroid or nodules), heavy menstrual periods, hair loss or coarseness, and slow heart rate. To treat hypothyroidism and raise thyroid hormone levels, a daily dose of Synthroid is typically prescribed long-term as a replacement.

• Standard Synthroid dosage for adults: 50-200 mcg once daily long term

• Maximum Synthroid dosage for adults: 300 mcg daily long term

RELATED: Hypothyroidism vs. hyperthyroidism

Synthroid dosage for thyroid-stimulating hormone (TSH) suppression

High doses of Synthroid are used in TSH-dependent thyroid cancer. The goal is to flood the body with the thyroid hormone and greatly reduce the TSH levels. The basis of this therapy is the knowledge that TSH is a growth factor for thyroid cancer. So, lower serum TSH levels should lead to decreased disease activity. 

The required dose is usually greater than 2 mcg per kg per day. The dose depends on many factors including the patient's age, weight, cardiovascular disease, other medical conditions (including pregnancy), and/or medications, co-administered food, etc. 

Synthroid dosage for children

Synthroid is FDA-approved for children of all ages to treat hypothyroidism and to lower TSH levels in thyroid cancer. Infants and children require higher doses per kilogram compared with adults. A provider will prescribe a dose depending on weight. The maximum dosage for children is 300 mcg per day.

• Standard Synthroid dosage for children: 25-300 mcg

• Maximum Synthroid dosage for children: 300 mcg per day 

Synthroid dosage restrictions

• Renally-impaired patients (kidney disease): Not specified

• Hepatically-impaired patients (liver disease): Not specified

• Coronary artery disease (heart disease): Start at 12.5-25 mcg/day, then adjust dose by 12.5-25 mcg/day every six to eight weeks.

• Newborns (0-3 months) at risk for heart failure: Start at a lower dose

• Children at risk for hyperactivity: Start at a lower dose, and increase on a weekly basis by one-fourth of the full recommended replacement dose until the full recommended replacement dose is reached.

• Pregnancy: It is safe to use Synthroid while pregnant and breastfeeding. The body requires more thyroid hormones during pregnancy. Pregnant women will require a 30% to 50% increase in dose. Synthroid dosage should be reduced to pre-pregnancy levels immediately after delivery and monitored closely while breastfeeding. 

• Seniors: Synthroid should be started at less than the full replacement dose in this population. Older adults may only require less than 1 mcg/kg/day.

Synthroid dosage for pets

Synthroid can be used off-label in cats and dogs. Animals can also have thyroid gland deficiencies that may lead to low thyroid hormone levels. Please follow your veterinarian’s directions very carefully as they may be significantly different from those for human use. 

THYRO-TABS CANINE (active ingredient: levothyroxine sodium tablets), is the only FDA-approved replacement therapy for hypothyroidism in dogs. It is dosed based on weight and given by mouth every 12 or 24 hours.

How to take Synthroid

Synthroid is taken as a tablet orally on an empty stomach. Take once a day, every day at the same time on an empty stomach (before breakfast or more than three hours after the last meal of the day). Wait 30 minutes to one hour before eating or drinking anything other than water.

• Take your medicine as directed. Your dose may need to be changed several times to find what works best for you. You may have to take this medicine for four to eight weeks before your symptoms start to get better.

• Read and follow the patient instructions that come with this medicine. Refer to your medication guide or talk to your doctor or pharmacist for instructions on how to take Synthroid.

• Take Synthroid at least four hours before or after taking medications or supplements known to interfere with its absorption.

• If you are giving this medicine to a baby or child who cannot swallow the tablet whole, crush the tablet and mix it in 1 to 2 teaspoons (5 to 10 mL) of water. Administer the mixture using a spoon or dropper. Do not mix the tablet with any other liquid except water. Do not store the mixture for future use.

• Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light.

• Don’t stop taking Synthroid without asking your doctor. You may need to taper doses before completely stopping the medicine.

Synthroid dosage FAQs

How long does it take Synthroid to work?

Synthroid takes about four to six weeks to see the maximum effect. It is taken long-term to maintain thyroid hormone levels in the body. Diseases of the gut like celiac disease and Helicobacter pylori infection can compromise the absorption of Synthroid. Higher doses will be required in these cases.

How long does Synthroid stay in your system?

Synthroid is primarily excreted through the kidneys. It is slowly eliminated and can take up to five weeks to leave the body.

What happens if I miss a dose of Synthroid?

Patients who miss a dose of Synthroid should take it as soon as they remember. However, if it is almost time for their next dose, patients should not take the missed dose in addition to a regular dose. In this case, wait until the next scheduled dose and take it on time. Do not take extra medicine to make up for a missed dose.

How do I stop taking Synthroid?

Do not suddenly stop taking this Synthroid without first checking with your doctor. Your doctor may want you to gradually reduce the amount you are using before stopping completely.

If Synthroid is not working well for you, your doctor may prescribe any of the following alternatives. The majority of these are available in both brand name and generic formulation. There are no over-the-counter alternatives.

• Armour Thyroid

• Cytomel

• Euthyrox

• Levoxyl

• Nature-Throid

• Np Thyroid

• Propylthiouracil

• Tapazole

• Tirosint

• Unithroid

• Westhroid

• Wp Thyroid

What is the maximum dosage for Synthroid?

The maximum dosage of Synthroid is 300 mcg per day

What are the side effects of Synthroid?

The thyroid hormone is a naturally occurring substance in the body so there should be no adverse effects experienced if a patient is euthyroid (thyroid hormone within normal range). However, in overtreatment (when the dose is too high), the patient will experience hyperthyroid symptoms or side effects such as:

• Increased heart rate

• Palpitations

• Sweating

• Weight loss

• Arrhythmias (irregular heartbeat)

Synthroid side effects may resolve after finding the right dosage. Severe side effects that require medical attention include chest pain, tremors, skin rash, shortness of breath, vomiting, diarrhea, changes in menstrual period, and fever.

Can Synthroid be used for weight loss?

Synthroid is ineffective and potentially dangerous when used for weight loss, especially in euthyroid patients. High doses can cause serious or even life-threatening effects. It should not be used in the treatment of obesity or for weight loss.

What interacts with Synthroid?

Synthroid should be taken on an empty stomach (30 minutes to one hour before breakfast) to ensure optimal absorption. Eating certain foods too close to your daily dosage can further decrease its absorption from the gut.

• Soy-based foods or soybean flours

• Cottonseed meal

• Grapefruit or grapefruit juice

• Walnuts

• High-fiber foods, like broccoli, cabbage, or kale

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Many medications can alter the effects of Synthroid, particularly antacids and proton pump inhibitors containing aluminum, magnesium, calcium carbonate, simethicone, sucralfate, or iron. Avoid taking Synthroid with any of the following:

• Cimetidine

• Dexlansoprazole

• Esomeprazole

• Famotidine

• Lansoprazole

• Nizatidine

• Omeprazole

• Pantoprazole

• Rabeprazole

Other Synthroid-drug interactions exist. Patients should inform their healthcare provider of all medications, supplements, and vitamins they’re taking. Even multivitamins containing calcium or iron can interfere with the absorption of Synthroid. Patients may be able to continue taking certain medications but they will need to be spaced out (typically four hours) from daily Synthroid doses.

Related resources for Synthroid dosages:

• Synthroid medication guide, Abbvie

• Synthroid, Epocrates

• A review of the pharmacokinetics of levothyroxine for the treatment of hypothyroidism, European Journal of Endocrinology

• Hypothyroidism in dogs, FDA

• Synthroid interactions, Epocrates

• Taking Synthroid the right way, Abbvie