Thiamine (Vitamin B1)
Thiamine (Vitamin B1) is a water-soluble vitamin essential for carbohydrate metabolism and normal neurologic function. **Key Clinical Points:** * **Deficiency Causes:** In small animals, deficiency typically results from inadequate dietary intake (e.g., unsupplemented all-meat diets, heat-destroyed commercial diets) or ingestion of thiaminase-containing foods (e.g., raw fish). In ruminants, it is often secondary to thiaminase-producing rumen bacteria or ingestion of thiaminase-containing plants like bracken fern, leading to **polioencephalomalacia (PEM)**. * **Clinical Signs:** Manifests as gastrointestinal issues (anorexia, salivation), severe neuromuscular/CNS signs (ataxia, seizures, ventroflexion of the neck in cats, loss of reflexes), or cardiac arrhythmias. * **Adjunctive Uses:** Frequently utilized in the management of **ethylene glycol toxicity** (to shunt metabolism away from toxic oxalates) and **lead poisoning**. * **Clinical Pearl:** Because the brain relies heavily on carbohydrate metabolism, the CNS is highly sensitive to thiamine deficiency, making rapid supplementation critical when neurologic signs appear.
Mechanism: Thiamine is actively transported into cells where it combines with adenosine triphosphate (ATP) to form **thiamine diphosphate (thiamine pyrophosphate or TPP)**. * **Enzymatic Co-factor:** TPP is an essential coenzyme for several key metabolic enzymes, including **pyruvate dehydrogenase**, **alpha-ketoglutarate dehydrogenase** (both critical for the Krebs cycle/aerobic energy production), and **transketolase** (pentose phosphate pathway). * **Metabolic Pathway:** Glucose โ Pyruvate. Without TPP, pyruvate cannot be converted into acetyl-CoA. * **Pathophysiology of Deficiency:** Diminished NADH production forces the cell into anaerobic glycolysis, converting accumulated pyruvic acid into lactic acid โ **Lactic acidosis** and cellular energy failure (especially in high-demand tissues like the brain and heart).
Dosing by species
- Polioencephalomalacia ยท Initially, 10 mg/kg IV; then, 10 mg/kg IM twice daily ยท IV, IM ยท twice daily ยท 2-3 days ยท If no improvement within 4 days, may be advisable to recommend slaughter.
- Thiamine deficiency ยท 20-200 mg ยท IM, SC, or IV ยท Not specified ยท Depending on formulation
- Polioencephalomalacia ยท Initially, 10 mg/kg IV; then, 10 mg/kg IM twice daily ยท IV, IM ยท twice daily ยท 2-3 days ยท If no improvement within 4 days, may be advisable to recommend slaughter.
- Thiamine deficiency ยท 100-250 mg ยท parenterally ยท twice a day ยท Experimentally, as little as 1 mg is effective
- Thiamine deficiency ยท 10-20 mg/kg ยท IM ยท Not specified ยท for several days
- Adjunctive treatment of hepatic lipidosis ยท 100 mg of thiamine in one liter of crystalloid fluids ยท IV ยท Not specified ยท With additional soluble B vitamins. Indicated if neck ventroflexion is present.
- Thiamine deficiency ยท 100-250 mg ยท SC ยท twice daily ยท for several days until regression of symptoms with complete recovery
- Thiamine deficiency ยท 10-20 mg/kg IM or SC two to three times daily until signs abate, then 10 mg/kg PO once daily ยท IM, SC, PO ยท two to three times daily, then once daily ยท 21 days for PO phase
Routes of administration
Contraindications
- Known hypersensitivity to thiamine or any of its components
- Known hypersensitivity to thiamine
Adverse effects
- Hypersensitivity or anaphylactic reactions (rare)
- Vasovagal anaphylactic response in cats (cardiac arrest, severe bradycardia, apnea, hypotension, collapse) when given SC (rare)
- Injection site tenderness or muscle soreness (IM)
- Pain at the injection site (IM/SC)
- Anaphylaxis or hypersensitivity reactions (primarily with rapid IV administration)
Drug interactions
- Neuromuscular blocking agents ยท Thiamine may enhance the activity of neuromuscular blocking agents; clinical significance is unknown. ยท minor
Monitoring
- Clinical efficacy (resolution of neurologic, gastrointestinal, or cardiac signs)
- Electrolyte levels (potassium, phosphate, magnesium) if treating conditions like feline hepatic lipidosis
- Resolution of clinical signs of thiamine deficiency (e.g., neurological improvement)
- Dietary intake and nutritional balance
Overdose
Very large doses of thiamine in laboratory animals have been associated with **neuromuscular or ganglionic blockade**, though the clinical significance in veterinary patients is unknown. * **Massive Doses:** May cause hypotension and respiratory depression. * **Lethal Dose:** A lethal dose of 350 mg/kg has been reported. * **Treatment:** Generally, no specific treatment is required for most overdoses due to its water-soluble nature and rapid renal excretion.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.