Levothyroxine sodium is a prescription medication. It is used to treat hypothyroidism (underactive thyroid gland). Levothyroxine works as thyroid hormone replacement therapy in people who don’t make enough thyroid hormones. A 100 mcg dose of levothyroxine is an average dose and is not considered a very high dose. Please continue reading to find out more about levothyroxine dosing.

Is 100 mcg of levothyroxine daily a high dose?

100 mcg of levothyroxine daily is not a high dose. Generally speaking, levothyroxine doses of 25-50 mcg are considered low, 75-100 mcg are considered average, and above 125 mcg are considered high doses.

The recommended full replacement dose of levothyroxine in adults is 1.6 mcg/kg/day. This means an adult who weighs 180 lbs (82 kg) will need to take 133 mcg per day for full replacement of thyroid hormone. 

Your doctor will start you on a lower dose and gradually increase your daily dose by 12.5 mcg to 25 mcg every 4-6 weeks until you reach normal thyroid hormone levels, or “euthyroid.” Your provider will order regular blood tests, including thyroid stimulating hormone (serum TSH levels). They will adjust your levothyroxine dosage based on serum TSH levels and your clinical status (symptom control).

What factors affect levothyroxine dosing?

Since levothyroxine dosing is weight-based, your actual body weight determines how much levothyroxine is appropriate. Besides body weight, there are other factors that affect levothyroxine dosing, such as:

• Age

• Condition being treated

• Severity of thyroid disorder

• Co-occurring medical conditions, such as cardiovascular disease (coronary artery disease, cardiac failure, cardiac arrhythmias or irregular heartbeat, adrenal insufficiency)

• Drug interactions (certain other medications can increase or decrease levothyroxine absorption)

• Food interactions (you should take levothyroxine on an empty stomach in the morning 30-60 minutes before breakfast)

What is the maximum dose of levothyroxine?

The maximum levothyroxine dose is typically 200 mcg to 300 mcg daily. Doses over 200 mcg/day are rarely needed. If your symptoms are not controlled on high doses of levothyroxine (300 mcg per day), your doctor will look at possible causes, such as missed doses, drug interactions, or decreased levothyroxine absorption due to gastrointestinal issues. 

Note: If levothyroxine is not working and has not provided relief from the symptoms of hypothyroidism, talk to your doctor about taking desiccated thyroid extract (DTE) such as Armour Thyroid. Levothyroxine contains thyroxine (T4), while DTE contains combination therapy (T4 and T3). The body converts T4 to T3. However, in some people, combination therapy with T4 and T3 (DTE) can be beneficial if peripheral tissues are not converting T4 to T3 as expected.

What is the usual adult dose of levothyroxine for hypothyroidism (underactive thyroid)?

The starting dose in adults with primary hypothyroidism (growth and puberty completed) is 1.6 mcg/kg/day. This can be increased by 12.5 mcg to 25 mcg per day at intervals of 4 to 6 weeks until serum TSH levels return to normal. The maintenance dose is decided based on symptom relief and the results of thyroid function tests.

Note: Congenital hypothyroidism is present since birth, while acquired hypothyroidism develops after birth. Hypothyroidism can be due to thyroid disease (primary hypothyroidism), pituitary disease (secondary or central hypothyroidism), or hypothalamic disease (tertiary hypothyroidism). Levothyroxine is used as replacement therapy in all these types of hypothyroidism.

What is the dose of thyroid hormone for older adults?

The starting levothyroxine dose in people over 50 years of age is usually lower than 1.6 mcg/kg/day. The same is true for people with cardiovascular disease or cardiac arrhythmias such as atrial fibrillation. The dose is adjusted every 6 to 8 weeks until serum concentrations of TSH return to normal.

What is the dose of thyroid hormones in children? 

The starting daily dose of levothyroxine in pediatric patients depends on the child’s age as follows:

• 0-3 months: 10-15 mcg/kg/day

• 3-6 months: 8-10 mcg/kg/day

• 6-12 months: 6-8 mcg/kg/day

• 1-5 years: 5-6 mcg/kg/day

• 6-12 years: 4-5 mcg/kg/day

• Above 12 years (puberty incomplete): 2-3 mcg/kg/day

• Above 12 years (puberty completed): 1.6 mcg/kg/day

Note: Pediatric patients at risk of heart failure are prescribed a lower starting dose. Also, children at risk of hyperactivity are given one-fourth of the recommended starting dose initially; this is increased at weekly intervals until the full replacement dose is reached.

What is the dose of thyroid hormone replacement therapy in pregnant women? 

In pregnant women who already have a diagnosis of hypothyroidism, the levothyroxine dose is adjusted to achieve the goal TSH level. If the TSH level is higher than the trimester-specific goal, the dose is increased by 12.5 mcg to 25 mcg per day at 4-week intervals. The dose is reduced back to the pre-pregnancy dose immediately after delivery.

If a pregnant woman has new onset hypothyroidism (she is diagnosed with hypothyroidism for the first time during pregnancy), the recommended dose of levothyroxine ranges between 1 mcg/kg/day and 1.6 mcg/kg/day depending on the TSH level.

Note: It is safe to take levothyroxine to treat hypothyroidism if you are nursing an infant with breast milk. Learn more about levothyroxine and pregnancy.

What is the levothyroxine dose for thyroid cancer? 

Levothyroxine is used, along with radioiodine therapy and surgery, to treat certain types of thyroid cancer. The goal of levothyroxine therapy in such patients is TSH suppression to levels below 0.1 international units per liter (IU/L). This usually requires levothyroxine doses of more than 2 mcg/kg/day. People with high-risk thyroid tumors may need greater TSH suppression and higher doses. 

Note: Levothyroxine is not used for TSH suppression in people with benign thyroid nodules and nontoxic diffuse goiter in iodine-sufficient patients. There are no clinical benefits, and overtreatment with levothyroxine can cause hyperthyroidism (overactive thyroid) symptoms.

What are the side effects of high doses of levothyroxine?

Common side effects of levothyroxine are listed below. Tell your doctor if they are severe or do not go away in a few days. It could be that your levothyroxine dose is too high and needs to be lowered. Your doctor may order blood tests and adjust your dose. 

• Weight gain or weight loss

• Changes in appetite

• Vomiting

• Diarrhea

• Headache

• Fever

• Irregular menstrual periods

• Heat intolerance or excessive sweating

• Hair loss

• Muscle cramps

• Joint pain

More serious adverse effects of levothyroxine include an allergic reaction. Stop taking levothyroxine and call your doctor immediately or seek emergency medical attention if you develop:

• Shortness of breath

• Difficulty breathing

• Wheezing

• Chest pain

• Skin rash, hives, itching, flushing

• Stomach pain and nausea

• Swelling of hands, lower legs, ankles, or feet

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References:

1. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=38af4e36-b26b-485d-a6f3-7fbcf6072a0f#

2. https://medlineplus.gov/druginfo/meds/a682461.html

3. https://www.btf-thyroid.org/breastfeeding-and-thyroid-disease#:

4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7360670/#:~