Levothyroxine

Brand names

• Alti-Thyroxine
• Armour Thyroid
• Eltroxin
• Euthroid [CD]
• Euthyrox
• Levo-T
• Levotabs
• Levothroid
• Levoxine
• Levoxyl
• L-Thyroxine
• Proloid
• Synthroid
• Synthrox
• Syroxine
• Thyroid USP
• Thyrolar [CD]

Usage

To treat patients with an underactive thyroid gland, goiter (enlarged thyroid gland), and benign and malignant (noncancerous and cancerous) thyroid nodules.

How it works

Levothyroxine acts in the body as a substitute for natural thyroid hormone.

Side effects

Serious

In rare instances, levothyroxine may cause severe headaches, skin rash, hives, rapid or irregular heartbeat, chest pain, or shortness of breath. These symptoms may signal an overdose or an allergic reaction. Seek emergency medical assistance immediately.

Common

No common side effects are associated with the use of levothyroxine.

Less common

Leg cramps, diarrhea, changes in menstrual cycle, changes in appetite, sweating. sensitivity to heat, shaking of the hands, fever, headache. insomnia, irritability, weight loss, vomiting, nervousness. These symptoms may indicate your dose needs adjustment by your doctor.

Possible interactions

Other medicaments:

Levothyroxine may increase the effects of:

• warfarin (Coumadin) and increase the risk of bleeding; decreased anticoagulant dose is usually needed. More frequent INR testing (prothrombin time or protime) are needed.

Levothyroxine may decrease the effects of:

• digoxin (Lanoxin), when correcting hypothyroidism; a larger dose of digoxin may be needed.

Levothyroxine taken concurrently with:

• antacids may cause decreased levothyroxine absorption and a decreased therapeutic effect.
• all antidiabetic medicaments (insulin and oral hypoglycemic agents) may require an increased dose to obtain proper control of blood sugar levels.
• benzodiazepines (Librium and others) can enhance the toxic or therapeutic effects of both medicaments.
• conjugated estrogens (Premarin) may require an increased levothyroxine dose.
• tricyclic antidepressants may cause an increase in activity of both medicaments.

The following medicaments may decrease the effects of levothyroxine:

• cholestyramine (Cuemid, Questran)-may reduce its absorption; intake of the two medicaments should be separated by 5 hours.
• colestipol (Colestid).
• iron salts-by decreasing absorption.
• lovastatin (Mevacor).
• phenytoin (Dilantin) or fosphenytoin (Cerebyx)-can increase levothyroxine removal (clearance).
• ritonavir (Norvir).
• sodium polystyrene sulfonate (Kayexalate).
• sucralfate (Carafate).

Foods:

Enteral formulas for nutrition support that contain soybeans may increase the fecal elimination of thyroxine.

Herbal medicines or minerals:

Horseradish root might worsen low thyroid (hypothyroidism) or blunt effectiveness of therapy. Calcium doses (if taken) should be separated from levothyroxine doses by four hours. Cabbage and iodine may worsen goiters and exacerbate hypothyroidism. Gamma oryzanol (extracted from rice bran oil) can lower thyroid stimulating hormone (TSH) and change test results. Tiratricol is a naturally occurring metabolite of thyroxine and triiodothyronine. In theory, tiratricol may enhance the adverse action and effects of bungleweed, wild thyme and balm leaf. Talk to your doctor before adding any herbal medicines to levothyroxine.

Exposure to heat:

This medicament may decrease individual tolerance to warm environments, increasing discomfort due to heat. Consult your physician if you develop symptoms of overdose during the warm months of the year.

Heavy exercise or exertion:

Use caution if you have angina (coronary artery disease). This medicament may increase the frequency or severity of angina during physical activity.

Discontinuation:

Must be taken continually on a regular schedule to correct thyroid deficiency. Never stop it without talking to your doctor.

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