Thyroxine (T4)
| Thyroxine (T4) |
| Availability Powder for injection: 200 mcg/vial in 6- and 10-ml vials, 500 mcg/vial in 6- and 10-ml vials Tablets: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg |
| Indications and dosages ➣ Hypothyroidism; treatment or prevention of euthyroid goiter Adults: For healthy adults younger than age 50 and those over age 50 who have recently been treated or undergone short-term therapy, start at full replacement dosage of 1.7 mcg/kg P.O. daily, given 30 minutes to 1 hour before breakfast. For patients older than age 50 or younger than age 50 with heart disease, 25 to 50 mcg P.O. daily, increased q 4 to 6 weeks. In severe hypothyroidism, initial dosage is 12.5 to 25 mcg P.O. daily, adjusted by 25 mcg daily q 2 to 4 weeks. For patients who can’t tolerate oral doses, adjust I.M. or I.V. dosage to roughly half of oral dosage. ➣ Congenital hypothyroidism Children older than age 12 who have completed puberty and growth: 1.7 mcg/kg P.O. daily Children older than age 12 who have not completed puberty and growth: Up to 150 mcg or 2 to 3 mcg/kg P.O. daily Children ages 6 to 12: 4 to 5 mcg/kg P.O. daily Children ages 1 to 5: 5 to 6 mcg/kg P.O. daily Infants ages 6 to 12 months: 6 to 8 mcg/kg P.O. daily Infants ages 3 to 6 months: 8 to 10 mcg/kg P.O. daily Infants up to 3 months old: 10 to 15 mcg/kg P.O. daily ➣ Myxedema coma or stupor Adults: 200 to 500 mcg I.V. as a solution containing 100 mcg/ml. Additional 100 to 300 mcg may be given on day 2 if significant improvement has not occurred. Convert to P.O. therapy when patient is clinically stable. Thyroid-stimulating hormone suppression in well-differentiated thyroid cancers and thyroid nodules Adults: Dosage individualized based on disease and patient |
Mechanism of Action
Replaces endogenous thyroid hormone, which may exert its physiologic effects by controlling DNA transcription and protein synthesis. Levothyroxine has all the following actions of endogenous thyroid hormone. The drug:
- Increases energy expenditure
- Accelerates the rate of cellular oxidation, which stimulates body tissue growth, maturation, and metabolism
- Regulates differentiation and proliferation of stem cells
- Aids in myelination of nerves and development of synaptic processes in the nervous system
- Regulates growth
- Decreases blood and hepatic cholesterol concentrations
- Enhances carbohydrate and protein metabolism, increasing gluconeogenesis and protein synthesis.
Contraindications
- Hypersensitivity to drug, its components, or tartrazine
- Acute myocardial infarction
- Thyrotoxicosis
- Adrenal insufficiency
Precautions:
- Cardiovascular disease, severe renal insufficiency, diabetes mellitus
- Elderly patients
- Pregnant or breastfeeding patients.
Administration
- Be aware that all dosages are highly individualized.
- Give tablets on an empty stomach 30 minutes to 1 hour before first meal of day.
- If patient can’t swallow tablets, crush them and sprinkle onto small amount of food, such as applesauce. For infants and children, dissolve tablets in small amount of water, nonsoybean formula, or breast milk and administer immediately
- Don’t give oral form within 4 hours of bile acid sequestrants or antacids.
- Reconstitute Synthroid powder for injection with 5 ml of 0.9% sodium chloride injection. Shake until clear and use immediately.
- For I.V. administration, give each 100 mcg over at least 1 minute.
- Be aware that the various levothyroxine preparations aren’t bioequivalent. Patient should consistently use same brand or generic product, with dosing based on weight, age, physical condition, and symptom duration.
- When drug is given for thyroidstimulating hormone (TSH) suppression test, TSH suppression level is not well established and radioactive iodine (131I) is given before and after treatment course.
Adverse reactions
- CNS: insomnia, irritability, nervousness, headache
- CV: tachycardia, angina pectoris, hypotension, hypertension, increased cardiac output, arrhythmias, cardiovascular collapse
- GI: vomiting, diarrhea, abdominal cramps
- GU: menstrual irregularities
- Metabolic: hyperthyroidism
- Musculoskeletal: accelerated bone maturation (in children), decreased bone density (in women on long-term therapy) Skin: alopecia (in children), diaphoresis
- Other: heat intolerance, weight loss
Patient monitoring
- Check vital signs and ECG routinely.
- Monitor thyroid and liver function tests.
- Evaluate for signs and symptoms of overdose, including those of hyperthyroidism (weight loss, cardiac symptoms, abdominal cramps).
- Monitor closely for drug efficacy.
- Check patients with Addison’s disease or diabetes mellitus for worsening of these conditions.
- Watch for signs and symptoms of bleeding tendency, especially in patients receiving anticoagulants concurrently
Patient teaching
- Explain that patient may require lifelong therapy and must undergo regular blood testing.
- Tell patient or parent to report adverse effects, including signs or symptoms of hyperthyroidism or hypothyroidism.
- Caution patient to avoid driving and other hazardous activities until he knows how drug affects concentration and alertness.
- Advise patient to avoid getting overheated, as in hot environments or during vigorous exercise.
- Tell parents that child being treated may lose hair during first few months of therapy. Reassure them that this effect usually is transient.
- As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs, tests, and foods mentioned above.
Nursing Considerations
- Administer levothyroxine tablets as a single daily dose 30 to 60 minutes before breakfast. If patient has difficulty swallowing, crush tablet and suspend in a small amount of water or food.
- To prevent decreased drug absorption, give oral levothyroxine at least 4 hours before or after aluminum- or magnesium containing antacids, bile acid sequestrants, calcium carbonate, cation exchange resins, cholestyramine, colestipol, ferrous sulfate, kayexalate, or sucralfate.
- Expect to give drug I.V. or I.M. if patient can’t take tablets. Be aware that drug shouldn’t be given subcutaneously.
- For I.V. use, reconstitute drug by adding 5 ml of normal saline solution.
- Monitor PT of patient who is receiving anticoagulants; she may require a dosage adjustment.
- Monitor blood glucose level of diabetic patient. Prescriber may reduce antidiabetic drug dosage as thyroid hormone level enters therapeutic range.
- Expect patient to undergo thyroid function tests regularly during levothyroxine therapy.
REFERENCES
- Robert Kizior, Keith Hodgson, Saunders Nursing Drug handbook,1st edition 2024, Elsevier Publications. ISBN-9780443116070
- McGraw Hill- Drug Handbook, Seventh Edition, 2013, McGraw Hill Education Publications,9780071799430.
- April Hazard, Cynthia Sanoski, Davi’s Drug Guide for Nurses -Sixteenth Edition 2019, FA Davis Company Publications,9780803669451.
- Jones and Bartlet, Pharmacology for Nurses, Second Edition, 2020, Jones and Bartlet Learning Publications, ISBN 9781284141986.
- Nursebro.com, Search – Nursebro
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