levothyroxine/liothyronine

Action and Spectrum

Actions and Spectrum: 

Actions: 

The rate at which the body uses energy when at rest, known as the basal metabolic rate or BMR, is influenced by levothyroxine. It improves how well oxygen is used and encourages cells to produce energy. Similar to levothyroxine, liothyronine affects basal metabolic rate, encouraging heat production and energy expenditure.  

Spectrum: 

The most widely used and conventional medicine for treating hypothyroidism is levothyroxine. It is intended to compensate for the thyroid gland’s inadequate synthesis of thyroid hormones. Although it is less frequently used as the main treatment for hypothyroidism, liothyronine may be used as an adjuvant in certain circumstances, particularly when there are difficulties converting T4 to T3. 

Drug Interaction

Adverse Reaction

Frequency not defined 

Dyspnea 

Fever 

Allergic skin reaction 

Anxiety 

Alopecia 

Menstrual disease 

Abdominal cramps 

Myalgia 

Ataxia 

Diaphoresis 

Weight loss 

Constipation 

Tremor 

Headache 

Pruritis 

Diarrhea 

Tremor of hands 

Insomnia 

Urticaria 

Nausea 

Increased appetite 

Vomiting 

Nervousness 

Black Box Warning

Black Box Warning: 

Obesity has been treated with drugs that contain thyroid hormone activity, either by themselves or in combination with other medicinal agents. Doses that fall within the range of daily hormonal requirements don’t work to help euthyroid patients lose weight.

Greater dosages have the potential to cause severe or potentially fatal toxic effects, especially when combined with sympathomimetic amines like those used for anorectic purposes. 

Contraindication / Caution

Contraindication/Caution: 

Contraindications 

• Hypersensitivity 
• Thyrotoxicosis untreated 
• Adrenal insufficiency uncorrected 

Cautions 

• Myxedema 
• Adrenal insufficiency 
• Cardiovascular disease 
• Diabetes mellitus 
• Geriatrics 
• Pregnancy 

Pregnancy / Lactation

Pregnancy consideration:  

No data is available regarding the administration of the drug during pregnancy. 

Breastfeeding warnings:  

No data is available regarding the excretion of drug in breast milk. 

Pregnancy category: A 

Category A: well-controlled and satisfactory studies show no risk to the fetus in the first or later trimester. 

Category B: there was no evidence of risk to the fetus in animal studies, and there were not enough studies on pregnant women. 

Category C: there was evidence of risk of adverse effects in animal reproduction studies, and no adequate evidence in human studies must take care of potential risks in pregnant women.    

Category D: adequate data with sufficient evidence of human fetal risk from various platforms, but despite the potential risk, and used only in emergency cases for potential benefits.    

Category X: Drugs listed in this category outweigh the risks over benefits. Hence these categories of drugs need to be avoided by pregnant women.    

Category N: No data is available for the drug under this category. 

Pharmacology

Pharmacology: 

Triiodothyronine (T3) is the main active ingredient. It can be changed from thyroxine (T4) and travels throughout the body to affect the development and growth of different tissues. The exact mechanism of action of thyroid hormone is unknown, but it is thought to act through controlling DNA transcription and protein synthesis.

Liotrix is a uniform mixture of synthetic T4 and T3 in a 4:1 ratio. It is involved in normal metabolism, growth, and development; it promotes gluconeogenesis, increases the utilization and mobilization of glycogen stores, stimulates protein synthesis, and raises basal metabolic rate.  

Pharmacodynamics: 

Thyroid hormone receptors (THRA, THRB) can dimerize with retinoid X receptors to control target gene transcription, which starts the effect of T3. T4 and T3 can either directly or indirectly prevent the release of thyrotropin (TSH, TSHB, and CGA) by preventing the secretion of thyrotropin-releasing hormone (TRH). 

Pharmacokinetics: 

Absorption 

The bioavailability is 80%/90% 

The time to achieve peak effect is 2-4 hours/1-2 hours 

Distribution 

Levothyroxine is highly attached to plasma proteins when it is absorbed, especially transthyretin, albumin, and thyroxine-binding globulin (TBG). Its distribution and availability to tissues are impacted by this binding.  Liothyronine is highly linked to plasma proteins, such as TBG, transthyretin, and albumin, just like levothyroxine is.  

Metabolism 

In the liver, levothyroxine undergoes very little metabolism. The parent chemical is used for the majority of its activities. Levothyroxine can be changed from its less active form, T3, to liothyronine (T3) in peripheral tissues by removing one iodine atom. There is not much hepatic metabolism of liothyronine. The parent chemical is used for the majority of its activities. 

Elimination and Excretion 

The half-life of levothyroxine/liothyronine is 6-7 days/1-2 days. 

Levothyroxine/liothyronine is mostly excreted through the urine, with a tiny amount also being expelled in the feces. 

Adminstartion

Administration: 

Take one daily, 30 to 60 minutes before meals, on an empty stomach. Typically, therapy starts at lower dosages and is titrated according to the patient’s cardiovascular health. 

Unless there is a suspicion of temporary hypothyroidism, in which case therapy can be stopped for two to eight weeks after the child becomes three years old in order to reevaluate the condition, therapy should be started as soon as possible following diagnosis and continued for life. 

In individuals who have maintained a normal TSH throughout those two to eight weeks, stopping therapy may be warranted. Within two to three weeks of treatment, TSH and T4 levels should be normal thanks to adequate medication. 

In the first four weeks of treatment, the dosage of thyroid hormone should be readjusted after doing appropriate clinical and laboratory assessments, which include measuring serum levels of T4, free and bound thyroid hormone, and TSH. 

Thyroid hormone dosage is dictated by the indication and needs to be customized in each case based on the patient’s response and test results. It is oral to administer thyroid hormones.

When oral medication is impractical or undesirable, as in the case of treating myxedema coma or during total parenteral feeding, injectable levothyroxine sodium may be administered intravenously in acute, emergency situations. Administration intramuscularly is not recommended due to reported low absorption. 

Patient Information Leaflet

Patient information leaflet 

Generic Name: levothyroxine/liothyronine 

Pronounced: lee-voh-thy-ROX-een/lee-oh-thy-ROH-neen 

Why do we use levothyroxine/liothyronine? 

Treatment for hypothyroidism, a disorder in which the thyroid gland does not produce adequate thyroid hormones, involves the use of levothyroxine and liothyronine.