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Cynomel (Liothyronine)

Active Ingredient: Liothyronine
Dosage: 25 mcg
Route of Administration: Oral
Dosage Form: Tablets
Quantity per package: 100
Availability: Out of stock

Cynomel is a medication for the treatment of severe hypothyroidism. This disorder appears when the body does not produce enough thyroid hormone.

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The medication is indicated in the treatment of hypothyroidism when there is a decrease or absence of thyroid function, caused by antithyroid agents, radiotherapy, primary atrophy, partial or total removal of the gland or due to functional alterations. It is effective in the treatment of hypothyroidism of any etiology. The use of Cynomel as a substitute therapy in simple goiter (non-toxic) results in a rapid reduction in the size of the thyroid gland. The drug has proven useful in some patients who do not respond to other thyroid preparations, probably because it has an efficient absorption. It can be used in patients allergic to dried thyroid or other porcine or bovine thyroid extracts. The drug can be used in the T3 suppression test to differentiate between hyperthyroidism and euthyroidism.

Dosage and administration

For oral use. The drug is usually given in a single dose, preferably before breakfast. In moderate hypothyroidism and reproductive disorders caused by hypothyroidism, the recommended starting dose is one daily tablet. The dose should be adjusted according to the response, with gradual increases on average to one tablet daily every 1 to 2 weeks. The usual maintenance dose is one to three tablets daily. When a thyroid therapy based on thyroid extract, L-thyroxine or thyroglobulin is discontinued and treatment with Cynomel is initiated, therapy can be initiated at low doses to gradually increase according to the patient’s response. When an initial dose is selected, it should be taken into account that this drug acts almost immediately and that the residual effects of the other thyroid preparation may persist several weeks after being administered. Similarly, although the drug can be discontinued rapidly, its metabolic effects persist for 72 hours. Special instructions for the T3 suppression test: when the absorption of the I131 drops to high levels, administer 3 to 4 tablets daily for 7 days; then repeat the ingestion test of I131. In the hyperthyroid patient, the ingestion of I131 for 24 hours will not be significantly affected. In the euthyroid patient, the absorption of I131 will decrease to less than 20%.

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  • Thyrotoxicosis;
  • Unstable angor;
  • Acute myocardial infarction;
  • Irreversible adrenal insufficiency.

Side effects

Excessive doses of thyroid hormones can cause symptoms of hyperthyroidism: excitability, weight loss, palpitations, arrhythmias, tachycardia, diarrhea, sweating, tremor, headache and heat intolerance. The effects do not occur immediately and symptoms may appear 1 to 2 weeks after the initial dose. The medication in these cases should be interrupted until the symptoms disappear, to restart it one or two days later with lower doses.


In case of overdose, patients can experience symptoms such as headache, irritability, nervousness, excessive sweating, tachycardia, increased intestinal motility. Cases of angina pectoris or congestive heart failure may be aggravated and a shock box may develop. Excessive overdose of liothyronine may cause symptoms that suggest a thyroid crisis and will produce manifestations of hyperthyroidism. In any of these cases, the treatment should be suspended for several days and then re-started with a lower dose. If the overdose is acute, the absorption of the medication with the induction of vomiting and gastric lavage should be avoided. The treatment in cases of shock includes supportive measures and the treatment for unrecognized adrenal insufficiency should be considered. To treat increased sympathetic activity, antiadrenergic medications may be administered.


The administration of Cynomel can generate pharmacological interactions with various drugs. Liothyronine increases the anticoagulant effects of warfarin and acenocoumarol. It can cause an increase in the requirements of insulin or oral hypoglycemic agents in diabetic patients. Cholestyramine hinders the absorption of thyroid hormones. The use of estrogen or oral contraceptives with estrogen may decrease free levothyroxine and therefore increase thyroid hormone requirements. The use of thyroid hormones together with imipramine and other tricyclic antidepressants may increase the antidepressant activity, as well as the activity of thyroid hormones. Thyroid hormones can enhance the toxic effects of digital; also, thyroid replacement therapy increases the metabolic rate, which may require an increase in the digital dose. It has been reported that concomitant administration of thyroid hormones and ketamine may cause hypertension and tachycardia. Thyroid hormones increase the adrenergic effect of catecholamines such as epinephrine and norepinephrine.