Levothyroxine | Dosages | Side Effects | Oral Use


Levothyroxine was first introduced in 1927 as an oral and injectable medication to treat thyroid hormone deficiency. It is a drug approved by the FDA that replaces a hormone (produced by the thyroid gland) to regulate an individual’s energy and metabolism system.

It is also known as L-thyroxine. In medical terms, thyroid hormonal deficiency is named ‘hypothyroidism’ refers to the condition of an individual in which their thyroid hormonal production level is less than the normal range.

Hypothyroidism occurs naturally or can be caused by surgical thyroid treatment or injury by medication/radiation. To diagnose hypothyroidism, there are the following symptoms that depend upon the thyroid health and efficient production of hormones;

  • Increased cholesterol levels
  • Depression
  • Fatigue
  • Hair loss 
  • Memory loss
  • Dry/rough skin
  • Constipation

For a healthy mental and physical life, an individual must have a sufficient number of thyroid hormones in the body. This medication is identified as the best oral and injectable medication to treat or prevent hormonal deficiency and goiter (enlarged thyroid gland). It may also treat and prevent certain thyroid tumors, cancer, and many more. Levothyroxine can be taken by mouth (oral) or by intravenous injection.

How to use Levothyroxine?

Oral Use of Levothyroxine 

  • First of all, the patient should read the ‘patient information leaflet’ provided by a pharmacist before taking thsi medicines and each time you get a refill.
  • Take these medicines exactly as prescribed by the doctor.
  • Swallow the tablet or capsule with a full glass of work and its oral works if a patient takes it on an empty stomach, 30 minutes to 1 hour before breakfast.
  • Swallow the tablet or capsule of levothyroxine whole. Do not split, crush, or chew. For infants or children who cannot swallow the whole tablet, give them crushed tablet into 1 to 2 teaspoons of water right away. Do not prepare it in advance.
  • Try to take the prescribed dosage of medicine daily at the same time as advised by the doctor. The levothyroxine dosage must be based on the patient’s age, weight, medical condition, laboratory test results, and response to treatment.
  • Keep using this medicine until your body responds to levothyroxine or with the consultation of your doctor. You may need to take this medicine for the rest of your life as thyroid replacement treatment takes years to cure.
  • Store at room temperature away from moisture and heat.
  • Injectable Use: Levothyroxine is given by injection to the patients who are unable to swallow or take it orally. The injection is given as an infusion into a vein.

Note about Levothyroxine 

  • Missed Dose: The patient must take medicine on time for an efficient and effective response. Do not miss the dose and never take two doses at one time.
  • Overdose: Seek emergency medical attention in case of overdose, having the following symptoms headache, leg cramps, tremors, feeling nervous or irritable, chest pain, shortness of breath, and fast or pounding heartbeats.
  • What to avoid: The patient taking Levothyroxine is advised to avoid the following food products during treatment; grapefruit juice, infant soy formula, soybean flour, cottonseed meal, walnuts, and high-fiber foods.

Side Effects of Levothyroxine:

The following side effects of using levothyroxine need emergency medical attention and doctor consultation.

  • Hives
  • Difficulty in breathing
  • Swelling on face, lips, tongue, or throat
  • Weight loss
  • Sweating
  • Anxiety
  • Trouble sleeping
  • Chest pain spreading to jaw or shoulder
  • Fever
  • Tremors
  • Weakness, tiredness, sleep problems (insomnia)
  • Memory problems, feeling depressed or irritable
  • Headache, leg cramps, muscle aches
  • Dryness of skin or hair
  • Hair loss
  • Irregular menstrual periods
  • Vomiting, diarrhea, or appetite changes
  • Skin rash
  • Sensitivity to heat

Dosages of Levothyroxine:

Levothyroxine dosage should be given as prescribed and advised by the doctor. While in general, the following dosage quantity must be taken into consideration:

Dosage consideration based on the form of medication and age of Patient:

Form of MedicationPatient’s ageDosage form and strengths
The tablet form of medication 

Adult and Pediatric Dosage Forms & Strengths

  • 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 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 consideration based on the intensity of hypothyroidism:

Intensity of HypothyroidismPatient’s ageDosage form and strengths




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 dose: 25-50 mcg/day

·       May adjust dose by 12.5-25 mcg q6-8Week

50 years with CV disease


·       Usual initial dose: 12.5-25 mcg PO qDay

·       May adjust dose by 12.5-25 mcg q4-6weeks until           the patient becomes normalized.

·       Dose range: 100-125 mcg PO qDay

Severe Hypothyroidism

 ·       Initial: 12.5-25 mcg PO qDay

·       Adjust dose by 25 mcg/day q2-4Week PRN

Subclinical Hypothyroidism


 ·       Initial: 1 mcg/kg PO qDay may be adequate, OR

·       If replacement therapy is not initiated, monitor            the patient annually for clinical status.

Warnings for patients and other people having certain health conditions:

Levothyroxine is not recommended to the following people who are already victimized of certain medical conditions and are advised to inform the doctor before prescribing levothyroxine to treat hormonal deficiency;

  • Heart Patients: For people having a history of severe heart problems, such as heart attack, abnormal heart rhythm, and heart failure.
  • Diabetic patients: Using levothyroxine for diabetic patients may worsen their health. The doctor should monitor blood sugar levels more closely before starting levothyroxine dosage.
  • Osteoporosis Patients: Osteoporosis patients can have a higher risk of bone fracture as their bone mineral density will decrease in the long-period usage of levothyroxine.
  • Patients of the adrenal or pituitary gland: If an individual has adrenal and pituitary gland problems, can have abrupt variation in the thyroid hormone level.
  • Patient of blood clotting disorders: Taking levothyroxine can cause heavy bleeding and put the patient’s life at higher risk.
  • Kidney disease: The doctor can prescribe levothyroxine to the Kidney patients in lower dosage as its increase can damage the kidney adversely.
  • Any other food or drug allergic patient: If the patient has an allergy to a particular food or drug, they should inform the doctor well before time to prescribe levothyroxine dose accordingly.
  • For pregnant women: Pregnant women can take levothyroxine as it was found perfectly safe, but the significant deficiency of hormone level could harm mother and fetus. Therefore, pregnant women should tell the doctor to prescribe levothyroxine dose accordingly.
  • For women who are breastfeeding: The usage of levothyroxine can be less effective for breastfeeding women as its amount can pass into breast milk. Ask your doctor about the safe way to take levothyroxine dose during breastfeeding.
  • For seniors: If the patient is older than 65 years, levothyroxine can put them at higher risk of heart problems. For this, the doctor should recommend the low dosage accordingly.
  • For children: Levothyroxine tablet has been found perfectly safe for children of all ages to treat hypothyroidism.

Avoid using Levothyroxine with certain drugs:

When an individual takes levothyroxine with certain drugs, it may not properly treat specific medical conditions. The doctor must increase or decrease the dose of levothyroxine to get the maximum benefit out of it. The list of drugs is given as below;

  • When levothyroxine is less effective
  • The antidepressant sertraline
  • Rifampin and anti-seizure drugs
  • Colesevelam, cholestyramine, colestipol, kayexalate, or sevelamer
  • Orlistat
  • Simethicone and antacids
  • Cancer drugs
  • When other drugs are less effective
  • Diabetes drugs
  • Digoxin
  • Theophylline


  • https://www.drugs.com/levothyroxine.html retrieved on 2nd December 2021
  • https://www.rxlist.com/consumer_levothyroxine/drugs-condition.htm retrieved on 2nd December 2021
  • https://www.medicalnewstoday.com/articles/levothyroxine#alternatives retrieved on 2nd December 2021
  • https://www.nhs.uk/medicines/levothyroxine/ retrieved on 2nd December 2021
  • https://reference.medscape.com/drug/synthroid-levoxyl-levothyroxine-342732 retrieved on 2nd December 2021
  • https://www.webmd.com/drugs/2/drug-1433-7074/levothyroxine-oral/levothyroxine-oral/details retrieved on 2nd December 2021
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