Products
Levothyroxine is commercially available in tablet and capsule form (Eltroxin, Euthyrox, Tirosint). It is also combined fixed with the thyroid hormone liothyronine (T3) (Novothyral). In 2018, an additional solution in monodoses was registered (Tirosint Solution). Bioequivalence is not always given between different preparations. Therefore, switching should be done only under medical supervision.
Structure and properties
Levothyroxine (C15H11I4NO4, Mr = 776.9 g/mol) corresponds to the endogenous thyroid hormone thyroxine (T4). It is produced synthetically. Levothyroxine is a tetraiodinated derivative of the amino acid tyrosine. In medicinal products, it is present as levothyroxine sodium, a faint brownish-yellow, fine, crystalline powder that is very slightly soluble in water.
Effects
Levothyroxine (ATC H03AA01) replaces the hormone thyroxine produced by the thyroid gland in hypothyroidism. It leads to an increase in energy metabolism, promotes the formation of heat, and has an effect on protein, carbohydrate, lipid, nucleic acid, and vitamin metabolism. The full effect is delayed within three weeks. Levothyroxine has a long half-life of up to one week. Levothyroxine is a prodrug and prohormone because its effects are mediated primarily by its metabolite T3 (triiodothyronine, liothyronine), which is formed as needed. The physiological release of thyroid hormones is regulated in the organism by the hypothalamus and the pituitary gland and is subject to feedback. Therefore, when high doses (e.g., 200 µg) are administered, suppression of endogenous TSH secretion occurs.
Indications
Levothyroxine is used primarily for the treatment of hypothyroidism. In addition, other indications exist (e.g., TSH suppression, thyroid malignancies, thyroid suppression testing, struma prophylaxis).
Abuse
Thyroid hormones are abused as slimming agents by overweight individuals with normal thyroid function because they increase fat loss and increase basal metabolic rate. Because of the potential adverse effects, they are not recommended.
Dosage
According to the drug label. Levothyroxine is taken once daily in the morning at least 30 minutes before breakfast, fasting with water. Do not take any medications, dietary supplements, coffee, or milk at the same time! The therapy is started gradually and the dose is determined individually. During pregnancy, the required dose may be increased.
Contraindications
- Hypersensitivity
- Fresh heart attack
- Heart muscle inflammation
- Inflammation of the layers of the heart
- Severe angina
- Heart failure with increased heart rate
- Untreated adrenal dysfunction.
- Untreated pituitary insufficiency
- Treatment of overweight and obesity in individuals without hypopituitarism.
For complete precautions, see the drug label.
Interactions
Levothyroxine has a high potential for interaction. Certain agents may decrease the absorption of thyroid hormone. These include ion-exchange resins, sevelamer, antacids, and calcium and iron supplements. Foods can also have an effect on absorption, for example soy products. Therefore, fasting intake is recommended.
- Enzyme inducers may decrease the effects of levothyroxine.
- Levothyroxine is highly protein-bound and can be displaced from protein binding by other agents (and vice versa).
In addition, other drug-drug interactions are possible.
Adverse effects
Levothyroxine is usually well tolerated in optimal settings. However, if the dose is too high or increased too quickly, adverse effects typical of hyperthyroidism may occur. These include, for example, nervousness, insomnia, diarrhea, tremors, sweating, headaches, weight loss, palpitations and cardiac arrhythmias.
