Focus on Pharm: Levothyroxine

Levothyroxine is synthetic thyroid hormone that goes by a lot of different brand names – common ones are Synthroid™, Tirosint™ and Levoxyl™. According to the website clincalc.com, levothyroxine is the second most prescribed medication in the United States between the years 2013 and 2019. So it is definitely a medication you will see on exams and at the bedside. 

To help you understand what you need to know about levothyroxine, let’s go through it using the Straight A Nursing DRRUGS framework. This drug is actually one of the more straightforward medications you’ll learn, so this should be pretty painless. 

D: Drug Class

Levothyroxine is in the therapeutic class of hormones and in the pharmacologic class of thyroid preparations. Essentially it is synthetic thyroid hormone, or more specifically, synthetic T4. The main effect of synthetic thyroid hormone is to increase the body’s metabolic rate. More specifically, thyroid hormone has the following roles:

• Plays a role in brain and CNS development
• Promotes gluconeogenesis and increased utilization of glycogen stores
• Stimulates the synthesis of proteins
• Encourages cells to grow and differentiate

R: Routes of Administration

Levothyroxine can be given PO as a tablet, capsule and oral solution, and it can also be administered IV.

The PO tablets come in 12 different strengths ranging from 25 mcg to 300 mcg in order to achieve easy and precise dosing.

 

R: Regular dose range

In adults, the general range for levothyroxine is 1.6 mcg/kg per day but this can be increased by 12.5 to 25 mcg per day every four to six weeks until the desired TSH level is reached and the patient stops showing signs of hypothyroidism. Note that the starting dose is essentially rounded to the nearest available tablet strength. For example, someone weighing 65 kg would technically have a dose of 104 mcg per day, but the dose calculator at synthroidpro.com suggests a dose of 100 mcg per day…there’s simply no tablet that contains only 4 mcg.

Older patients and those with cardiac disease are recommended to start more conservatively at 12.5 to 25 mcg/day with dose adjustments occurring every six weeks instead of every four. 

U: Uses for Levothyroxine

Levothyroxine is mainly used to treat hypothyroidism and myxedema coma, but it is also used to suppress the growth of thyroid cancer and suppress euthyroid goiters (also called a nontoxic goiter). This is a goiter not related to hypothyroidism, hyperthyroidism, cancer or inflammation.

G: Guidelines for administration

Safety: 

Levothyroxine is not to be used in patients with a recent MI as its effect on the SNS could be harmful to an already compromised heart. For this reason it is used with caution in individuals with cardiac disease and these patients may start therapy at lower doses and titrate up more slowly. It is also used cautiously in individuals with severe renal impairment. 

Infants with hypothyroidism will receive levothyroxine, but they must be monitored closely for cardiac arrhythmias and overload as well as aspiration when therapy is initiated and for the first two weeks. 

Levothyroxine has many drug-drug interactions. Some notable ones are: 

• It can decrease the effectiveness of digoxin.
• If the patient is taking an adrenergic medication such as phenylephrine or dopamine, it can increase the cardiovascular effects of those drugs.
• It can increase the effects of warfarin, putting your patient at risk for bleeding.
• In diabetic patients it can increase the amount of insulin or oral hypoglycemic that is needed to keep blood sugars within the desired range.
• If used concurrently with ketamine, it can cause significant tachycardia and hypertension.
• The absorption of levothyroxine can be decreased if the patient is also taking a PPI, antacid, or sucralfate as well as phosphate binders such as you see in chronic kidney disease.

Another important safety note is around misuse. Because levothyroxine does increase the metabolic rate, there is potential for it to be misused as a weight loss tool. Taking too much levothyroxine can have serious side effects that can be life-threatening and this medication absolutely should not be utilized as a “way to lose weight.”

Administration:

PO levothyroxine should be administered on an empty stomach to increase absorption. Foods or supplements that contain magnesium, calcium, iron or zinc can bind the medication and prevent it from absorbing adequately, so teach your patients to space these out by about four hours. Another culprit food is grapefruit juice, which can delay the absorption of levothyroxine. Interestingly, a study conducted in 2005 showed the effects of grapefruit juice on levothyroxine absorption were minimal but if the MD, pharmacist or nurse practitioner advises your patient to avoid grapefruit juice, then they should definitely follow their advice.

Teach your patient to take their levothyroxine tablet on an empty stomach with a full glass of water first thing in the morning, 30-60 minutes before breakfast. 

In the clinical setting you may be administering IV levothyroxine to your patients who are NPO and it should be given at a rate not to exceed 100 mcg over 1 minute.

Monitoring:

Because levothyroxine increases metabolism, monitor your patients for signs of HYPERthyroidism – these include tachycardia, angina, heat intolerance, tremors, headache, irritability, diarrhea, vomiting and insomnia…to name just a few! 

Your patient taking levothyroxine will have their TSH level checked regularly during initiation and titration of treatment and then periodically to ensure they are on the adequate dose.

S: Side Effects of Levothyroxine

In general, the side effects associated with levothyroxine are increased metabolism and can be related to dosing levels that are too high. These include tachycardia, heat intolerance, anxiety, GI upset, insomnia, headache and tremors.

I hope this brief overview of levothyroxine helps you perform well on your exams and have more confidence when working with patients taking this very common medication. 

For more articles on endocrine disorders, check out the list here.

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REFERENCES

AbbVie Inc. (n.d.). Dosing & Administration—SYNTHROID® (levothyroxine sodium tablets). https://www.synthroidpro.com/dosing

ClinCalc.Com. (n.d.). Levothyroxine—Drug Usage Statistics, ClinCalc DrugStats Database. Retrieved May 13, 2022, from https://clincalc.com/DrugStats/Drugs/Levothyroxine

Davis’s Drug Gide. (n.d.). Levothyroxine (Euthyrox, Levo-T). https://www.drugguide.com/ddo/view/Davis-Drug-Guide/109303/all/levothyroxine?refer=true

Johns Hopkins Medicine. (2019, November 19). Thyroid Hormone Replacement Therapy. https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/thyroid-hormone-replacement-therapy

Lilja, J. J., Laitinen, K., & Neuvonen, P. J. (2005). Effects of grapefruit juice on the absorption of levothyroxine. British Journal of Clinical Pharmacology, 60(3), 337–341. https://doi.org/10.1111/j.1365-2125.2005.02433.x