Levothyroxine
Levothyroxine (l-Thyroxine or T4) is a synthetic form of the naturally occurring thyroid hormone thyroxine. It is the drug of choice for the treatment of **hypothyroidism** in veterinary medicine. > **Clinical Warning:** Cases with pre-existing cardiac disorders require lower doses initially to prevent excessive cardiac stimulation. It can also unmask Addison's disease in patients with autoimmune polyglandular syndrome.
Mecanismo: Levothyroxine acts as a replacement for endogenous thyroxine. It enters cells and is converted to the more active T3 (triiodothyronine). T3 then **binds to specific intracellular thyroid hormone receptors** in the nucleus → **alters gene expression** → regulates metabolism, growth, and development.
Dosificación por especie
- Hypothyroidism · 0.02-0.04 mg/kg/day OR 0.5 mg/m2 body surface area daily · PO · sid or divided bid · Lifelong · Dose given with food. Monitor serum T4 levels pre-dosing and 4-8 hours after dosing.
- Hypothyroidism · 0.02-0.04 mg/kg/day OR 0.5 mg/m2 body surface area daily · PO · sid or divided bid · Lifelong · Dose given with food. Monitor serum T4 levels pre-dosing and 4-8 hours after dosing.
Las dosis son una referencia clínica para médicos veterinarios. Confirme siempre con la información vigente del producto y el paciente individual.
Vías de administración
Contraindicaciones
- Uncorrected adrenal insufficiency (Addison's disease)
- Acute myocardial infarction (general precaution)
Efectos adversos
- Tachycardia
- Excitability
- Nervousness
- Excessive panting
- Unmasking of Addison's disease
Interacciones farmacológicas
- Catecholamines · Actions of catecholamines are enhanced by thyroxine · moderate
- Sympathomimetics · Actions of sympathomimetics are enhanced by thyroxine · moderate
- Insulin · Altered insulin requirements; monitor carefully during initiation of therapy · moderate
- Oestrogens · May increase thyroid requirements by increasing thyroxine-binding globulin · moderate
- Ciclosporin · Therapeutic effect may be reduced by thyroid hormones · moderate
- Digoxin · Therapeutic effect may be reduced by thyroid hormones · moderate
- Digitoxin · Therapeutic effect may be reduced by thyroid hormones · moderate
- Ketamine · Tachycardia and hypertension may develop when given to patients receiving thyroid hormones · major
Monitoreo
- Serum total T4 levels (pre-pill trough and 4-8 hours post-pill peak)
- Clinical response (resolution of lethargy, weight gain, dermatological signs)
- Signs of thyrotoxicosis (panting, tachycardia, hyperactivity)
- Blood glucose in diabetic patients
Sobredosis
Clinical signs of overdosage (thyrotoxicosis) include **tachycardia**, **excitability**, **nervousness**, and **excessive panting**. Treatment involves withholding the medication until signs resolve, then restarting at a lower dose. Acute massive overdose may require gastrointestinal decontamination and supportive care (e.g., beta-blockers for severe tachycardia).
La referencia de fármacos de VetSheet está destinada a médicos veterinarios como apoyo a la decisión clínica; no sustituye el juicio profesional ni la información vigente del fabricante.