A Nurse’s Guide to Levothyroxine Dosages

• Patients with an underactive thyroid gland or hypothyroidism need synthetic thyroid hormones for the body to carry out physiological activities. That’s done with proper levothyroxine dosages. 
• Levothyroxine is available only through prescription. 
• Patients should understand that thyroid hormone therapy is lifelong. 

Your patients with an underactive thyroid gland or hypothyroidism need synthetic thyroid hormones for the body to carry out physiological activities. That’s done with the help of levothyroxine, available only through a prescription as a tablet or liquid.

Levothyroxine is a synthetic form of the thyroid hormone thyroxine that prevents symptoms of hypothyroidism and replaces the thyroxine hormone.

Ensure you do not confuse levothyroxine with levoFLOXacin, liothyronine, or Synthroid with Symmetrel. Levothyroxine should never be given for weight loss or combined with anorectic drugs to shed extra pounds, as it can cause life-threatening, toxic effects when given for these conditions.

Levothyroxine is available with liothyronine, or T3 hormone, as Thyrolar. It’s commonly used orally to treat hypothyroidism and pituitary thyroid-stimulating hormone (TSH) suppression and given intravenously to treat myxedema coma. Off-label levothyroxine is used to manage hemodynamically unstable potential organ donors. 

Levothyroxine use is contraindicated in patients sensitive to it, acute myocardial infarction, uncorrected adrenal insufficiency, and untreated subclinical or overt thyrotoxicosis.  

Exercise caution with its administration in the elderly population, as well as patients who have angina pectoris, hypertension or other cardiovascular diseases, adrenal insufficiency, diabetes mellitus, diabetes insipidus, myxedema, or swallowing disorders. 

Action of Levothyroxine

Levothyroxine converts to T3 and binds to thyroid receptor proteins, exerting metabolic effects by controlling DNA transcription and protein synthesis. Its therapeutic effects involve regulating normal metabolism, growth, and development. It also plays a role in increasing basal metabolism, enhancing gluconeogenesis, and stimulating protein synthesis.

The pharmacokinetics of levothyroxine are variable, resulting in incomplete absorption from the gastrointestinal tract. Protein binding is more than 99%, and it is widely distributed with minimal metabolism in the liver and de-iodinated in peripheral tissues. Levothyroxine is eliminated primarily by biliary excretion, having a half-life of six to seven days.  

The elderly may be sensitive to levothyroxine’s action, so dose adjustments are required. Moreover, it does not cross the placenta and has minimal distribution in breast milk. However, it can be given to all age groups depending on thyroid function.  

Drug Interactions

Levothyroxine interacts with cholestyramine, colestipol, aluminum— and magnesium-containing antacids, and calcium. Iron may decrease the absorption of levothyroxine, so a four-hour gap should be given between iron and levothyroxine intake. Estrogen may lower the therapeutic effects of levothyroxine. Therefore, their concomitant administration should be avoided

Levothyroxine may enhance the effects of oral anticoagulants, such as warfarin, resulting in increased chances of bleeding. If they are administered together, dose adjustments are required.

Levothyroxine Dosages

The dose of levothyroxine given orally in adults 60 years old or younger, without evidence of coronary heart disease, is 1.6 mcg/kg/day as a single daily dose. Dose adjustment is required by 12.5 to 25 mcg/day every three to six weeks. For adults older than 60 without evidence of coronary heart disease, the initial dose of levothyroxine is 12.5 to 50 mcg/day. Dose adjustment is required by 12.5 to 25 mcg/day at six- to eight-week intervals.

For children older than 12, when given levothyroxine for incomplete growth or puberty, the dose is 2 to 3 mcg/kg/day. For children between 6 and 12 years, the dose is 4 to 5 mcg/kg/day; for children ages 1 to 5, it is 5 to 6 mcg/kg/day. For children between six to 12 months, the dose of levothyroxine is 6 to 8 mcg/kg/day. For infants aged 3 to 5 months, the dose is 8 to 10 mcg/kg/day, and for those younger than three months, it is 10 to 15 mcg/kg/day.

For myxedema coma, levothyroxine is given orally to adults and older people, initially as 200 to 400 mcg, then 50 to 100 mcg once daily until able to switch to oral therapy.

For pituitary thyroid stimulating hormone suppression, levothyroxine is given orally to adults and the elderly in doses greater than two mcg/kg/day, usually required to suppress TSH below 0.1 milliunits/L.

Side Effects and Adverse Effects of Levothyroxine 

Occasional side effects of levothyroxine include reversible hair loss at the start of the therapy in children. Rare side effects include dry skin, rash, urticaria, gastrointestinal intolerance, pseudotumor cerebri, and severe headache in children.

Adverse effects of levothyroxine occur when it is given in excessive doses, more than required, producing symptoms of hyperthyroidism, such as weight loss, palpitations, increased appetite, tremors, anxiety, tachycardia, hypertension, headache, insomnia, and menstrual irregularities. Long-term therapy can also decrease bone mineral density.

Nursing Considerations

Do not change brands to administer levothyroxine through the intravenous route, as there are known issues with bioequivalence between manufacturers. For IV reconstitution, reconstitute a 200 mcg or 500 mcg vial with 5 mL 0.9% NaCl to provide a concentration of 40 or 100 mcg/mL, respectively; shake until clear. Administer the solution immediately and discard unused portions. Give each 100 mcg or less over 1 minute and store the vials at room temperature.

Levothyroxine is available in capsules by the brand name Tirosint in the strengths 13 mcg, 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg. It is also available as injection powder for reconstitution in the strengths 100 mcg, 200 mcg, and 500 mcg, and tablets in the strengths 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg.

Before starting levothyroxine therapy, obtain baseline TSH, T3, and T4 with weight and vital signs. Check for the signs and symptoms of diabetes, diabetes insipidus, hypopituitarism, and adrenal insufficiency. Treat with adrenocortical steroids before thyroid therapy in coexisting hypothyroidism and hypoadrenalism.

With levothyroxine therapy, monitor the patient’s pulse rate and rhythm. Report to the physician if the pulse is more significant than 100 or if you notice a marked increase. Observe the patient for anxiety and tremors and assess appetite and sleep pattern. Undertreatment in children may result in decreased intellectual development and linear growth, while overtreatment affects brain maturation and accelerates bone age.

Inform the patient not to discontinue drug therapy and that thyroid hormone replacement is lifelong. Please encourage them to attend follow-up visits and remind them that thyroid function tests are essential. Tell your patients to take the medication simultaneously daily, preferably in the morning before breakfast.

Monitor the patient’s pulse rate and rhythm and report any irregularity to the physician. Promptly report weight loss, chest pain, insomnia, anxiety, and tremors. In children, levothyroxine may cause reversible hair loss and increased aggressiveness during the first few months of therapy. It can take up to three weeks for the full therapeutic effects to show.

The Bottom Line

Levothyroxine is an essential drug used to treat an underactive thyroid gland or hypothyroidism. Tell the patient that thyroid hormone therapy is lifelong. Compel them to attend follow-up visits, get their thyroid function tests regularly, and inform you if any side effects occur with levothyroxine therapy. 

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