Thyroid-L
Medication Facts
- Performance enhancing
- Conditonally, Illicit Use
- Total hours to clear
- 30
- Frequently used
- Most Prescribed
- ESC ID
- THYROI-0867
- Trade names
- Not recorded
- Drug type
- Hormone
- Plasma half-life · hours
- 6 to 12 hours
- Equine dosing
- ESC review required
- Cmax · peak plasma concentration
- Not recorded
- Tmax · time to peak plasma concentration
- 2 to 5 hours
- ESC concentration guidance
- CMAX or 100% of Efficacy
- Primary withdrawal · hours
- 0
- Washout withdrawal · hours
- 0
- ESC control type
- A1 - Generally Controlled - Therapeutic
- Medication report required
- Generally not required, however understand exceptions.
Uses & side effects
Synthetc thyroid treats insulin resistance
Drug interactions
Levothyroxine sodium (Thyro-L) interacts with several medications and supplements commonly given to horses, which can either reduce the effectiveness of the treatment or cause dangerous cardiovascular and metabolic shifts.Absorption Inhibitors (Gastrointestinal Interactions)Oral binders given too close to a Thyro-L dose will severely block levothyroxine absorption in the equine gut:Sucralfate: Frequently used to treat equine gastric ulcers; it coats the stomach and binds directly to levothyroxine, rendering it ineffective.Oral Antacids & Mineral Supplements: Products containing aluminum, magnesium, calcium, or ferrous sulfate (iron) inhibit thyroid hormone uptake.High-Fiber Feeds: Large amounts of dietary fiber or beet pulp fed simultaneously can trap the medication and reduce its bioavailability.Clinical Action: Administer Thyro-L at least 2 hours before or 4 hours after these GI medications and supplements.Enhanced Drug Effects (Potentiation)Levothyroxine increases the body's sensitivity to certain emergency and therapeutic drugs:Sympathomimetics (Epinephrine & Norepinephrine): Thyro-L amplifies the effects of catecholamines. Co-administration sharply increases the risk of severe tachycardia, cardiac arrhythmias, and acute hypertension.Warfarin (Anticoagants): Thyroid hormones enhance the activity of oral anticoagulants by accelerating the degradation of vitamin K-dependent clotting factors, increasing the horse's risk of hemorrhaging.Reduced Drug Effects (Antagonism)Digoxin: Levothyroxine increases the metabolic clearance and alters the distribution of digoxin. If a horse is being treated for heart failure, its digoxin dose may need to be increased to maintain therapeutic levels.Insulin & Anti-Diabetic Agents: While Thyro-L is often used to treat Equine Metabolic Syndrome (EMS), shifting a horse's metabolic rate can alter glucose dynamics, potentially increasing overall insulin requirements in diabetic or severely insulin-dysregulated animals.Hormone & Steroid InteractionsCorticosteroids (Dexamethasone): Long-term steroid use suppresses the pituitary gland's secretion of Thyroid Stimulating Hormone (TSH) and can blunt the horse’s systemic response to thyroid therapy.Estrogens: Supplemental estrogens can increase thyroid-binding globulin levels, leaving less "free" (active) T4 circulating in the bloodstream and increasing the horse's dosage needs.Pergolide Mesylate (Prascend): Highly protein-bound drugs used for PPID (Cushing's) can temporarily displace thyroid hormones from plasma proteins. While studies show it does not damage the thyroid gland's core function, it can blur baseline diagnostic blood tests.Cardiovascular Risk MultipliersRecent studies from the Journal of Veterinary Cardiology warn that levothyroxine supplementation significantly increases the baseline risk of atrial fibrillation and cardiac thickening in horses. Combining Thyro-L with other medications that stress the heart (like certain anesthetics, ketamine, or continuous beta-agonists) requires extreme veterinary oversight