Liothyronine sodium is the L-isomer of triiodothyronine

Triiodothyronine

Triiodothyronine, C15H12I3NO4, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate....

 (T₃), a form of thyroid hormone

Thyroid hormone

The thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine ,...

 used to treat hypothyroidism

Hypothyroidism

Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone.Iodine deficiency is the most common cause of hypothyroidism worldwide but it can be caused by other causes such as several conditions of the thyroid gland or, less commonly, the pituitary gland or...

 and myxedema coma

Myxedema coma

Myxedema coma is state of decompensated hypothyroidism. The patient may have lab values identical to a "normal" hypothyroid state, but a stressful event precipitates the myxedema coma state. Primary symptoms of myxedema coma are altered mental status and hypothermia...

. It is marketed under the brand name Cytomel (or Tertroxin in Australia).

Pharmacology

Liothyronine is the most potent form of thyroid hormone. As such, it acts on the body to increase the basal metabolic rate, affect protein synthesis and increase the body's sensitivity to catecholamines (such as adrenaline) by permissiveness. The thyroid hormones are essential to proper development and differentiation of all cells of the human body. These hormones also regulate protein, fat, and carbohydrate metabolism, affecting how human cells use energetic compounds.

In comparison to levothyroxine

Levothyroxine

Levothyroxine, also L-thyroxine, synthetic T4, or 3,5,3',5'-tetraiodo-L-thyronine, is a synthetic form of thyroxine , used as a hormone replacement for patients with thyroid problems. The natural hormone is chemically in the chiral L-form, as is the pharmaceutical agent...

 (T₄), liothyronine has a faster onset of action as well as a shorter biological half-life

Biological half-life

The biological half-life or elimination half-life of a substance is the time it takes for a substance to lose half of its pharmacologic, physiologic, or radiologic activity, as per the MeSH definition...

, which may be due to less plasma protein binding

Plasma protein binding

A drug's efficiency may be affected by the degree to which it binds to the proteins within blood plasma. The less bound a drug is, the more efficiently it can traverse cell membranes or diffuse. Common blood proteins that drugs bind to are human serum albumin, lipoprotein, glycoprotein, α, β‚ and γ...

 to thyroxine-binding globulin

Thyroxine-binding globulin

Thyroxine-binding globulin binds thyroid hormone in circulation. It is one of three proteins responsible for carrying the thyroid hormones thyroxine and 3,5,3’-triiodothyronine in the bloodstream. Of these three proteins, TBG has the highest affinity for T4 and T3, but is present in the lowest...

 and transthyretin

Transthyretin

Transthyretin is a serum and cerebrospinal fluid carrier of the thyroid hormone thyroxine and retinol binding protein bound to retinol. This is how transthyretin gained its name, transports thyroxine and retinol...

.

Side effects

Liothyronine may cause a number of side effects, mostly similar to symptoms of hyperthyroidism

Hyperthyroidism

Hyperthyroidism is the term for overactive tissue within the thyroid gland causing an overproduction of thyroid hormones . Hyperthyroidism is thus a cause of thyrotoxicosis, the clinical condition of increased thyroid hormones in the blood. Hyperthyroidism and thyrotoxicosis are not synonymous...

, which include:

• weight loss
• tremor
• headache
• upset stomach
• vomiting
• diarrhea
• stomach cramps
• nervousness
• irritability
• insomnia
• excessive sweating
• increased appetite
• fever
• changes in menstrual cycle
• sensitivity to heat

Black box warning

The package insert for Cytomel contains the following black box warning

Black box warning

In the United States, a black box warning is a type of warning that appears on the package insert for prescription drugs that may cause serious adverse effects...

:

Drugs with thyroid hormone activity, alone or together with other therapeutic agents, have been used
for the treatment of obesity. In euthyroid patients, doses within the range of daily hormonal
requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amine

Sympathomimetic amine

Sympathomimetic drugs mimic the effects of transmitter substances of the sympathetic nervous system such as catecholamines, epinephrine , norepinephrine , dopamine, etc...

s such as those used for their anorectic effects.

Uses

Physicians can use this instead of levothyroxine (T4) for patients undergoing thyroid withdrawal. When a patient has thyroid cancer or Grave's, I-131 ablation therapy can be used to remove any trace thyroid tissue. In order for I-131 therapy to be effective, the trace thyroid tissue must be avid to iodine. The best method is to starve the tissue of iodine but this can lead to hypothyroid symptoms for the patient. Withdrawal from levothyroxine can be done but it takes 6 weeks of withdrawal for the remaining thyroid tissue to be completely starved. 6 weeks is needed owing to levothyroxine's long half life. 6 weeks can be inconvenient for the patient and delay treatment. Liothyronine instead can be taken and withdrawn from for 2 weeks to starve the thyroid tissue. This is much safer and more convenient than levothyroxine.