Thyrotropin — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	IV	0.1 IU/kg (or 5 IU per dog, up to 10 IU)	Single dose	Duration: One-time administration for diagnostic testing Notes: Collect blood samples for T4 measurement before and 6 hours after TSH administration. Some protocols use 0.1 IU/kg IV.
Cat	IV	0.1 IU/kg (or 1-2 IU per cat)	Single dose	Duration: One-time administration for diagnostic testing Notes: Collect blood samples for T4 measurement before and 6 hours after TSH administration. Use is limited due to availability and cost.
Horse	IV	5 IU per horse (or 0.1 IU/kg)	Single dose	Duration: One-time administration for diagnostic testing Notes: Collect blood samples for T4 measurement before and 4-6 hours after TSH administration.
Cattle	IV	0.1 IU/kg	Single dose	Duration: One-time administration for diagnostic testing Notes: Not commonly used; may be used in research.
Small Ruminants	IV	0.1 IU/kg	Single dose	Duration: One-time administration for diagnostic testing Notes: Not commonly used; may be used in research.
Rabbit	IV	0.1 IU/kg	Single dose	Duration: One-time administration for diagnostic testing Notes: Not commonly used; may be used in research.
Bird/Poultry	IV	0.1 IU/kg	Single dose	Duration: One-time administration for diagnostic testing Notes: Not commonly used; may be used in research.

Clinical Indications & Species Uses

General Indications

Diagnostic aid for thyroid function testing

Dog (Canine)

Diagnosis of hypothyroidism (TSH stimulation test)
Differentiation of primary vs secondary hypothyroidism

Cat (Feline)

Diagnosis of hypothyroidism (rarely used; may be used in research or specific cases)

Horse (Equine)

Diagnosis of hypothyroidism (TSH stimulation test)

Pharmacology & Mechanism of Action

Drug Class: Hormone | Pharmacological Group: Anterior Pituitary Hormone

Mechanism of Action: Thyrotropin (thyroid-stimulating hormone, TSH) is a glycoprotein hormone produced by the anterior pituitary. It binds to TSH receptors on thyroid follicular cells, activating adenylate cyclase and increasing intracellular cyclic AMP. This stimulates thyroid hormone synthesis and secretion, including thyroxine (T4) and triiodothyronine (T3). In diagnostic use, exogenous thyrotropin is administered to test thyroid function and differentiate primary from secondary hypothyroidism.

Pharmacodynamics: Thyrotropin increases the uptake of iodine by the thyroid gland, stimulates the synthesis and release of T4 and T3, and promotes thyroid cell growth. In normal animals, administration of thyrotropin results in a measurable increase in serum T4 concentrations within hours. In animals with primary hypothyroidism, the response is blunted or absent; in secondary hypothyroidism, a normal response may be seen. The drug also has effects on thyroidal blood flow and metabolism.

⚡ Pharmacokinetics Summary

Absorption: Thyrotropin is a peptide hormone and is not orally bioavailable; it must be administered parenterally (IV, IM, or SC). Absorption after IM or SC injection is rapid, with peak serum levels achieved within 1-2 hours.

Distribution: Thyrotropin is distributed throughout the extracellular fluid. It does not cross the blood-brain barrier to a significant extent. Its volume of distribution is small, approximating the plasma volume.

Metabolism: Thyrotropin is metabolized by proteolytic enzymes in the liver and kidneys. The metabolic clearance is rapid, with a half-life of approximately 1-2 hours in most species.

Excretion: The metabolites are excreted in the urine. A small amount of intact thyrotropin may be excreted in the urine.

Half-Life: Dogs: ~1-2 hours; Cats: ~1-2 hours; Horses: ~1-2 hours; Cattle: ~1-2 hours

Bioavailability: Not orally bioavailable; parenteral administration results in complete bioavailability.

Protein Binding: Minimal protein binding (<10%)

Available Formulations & Strengths

Injectable Solution (lyophilized powder for reconstitution) Typically 10 IU/mL after reconstitution (IV, IM, SC)

Contraindications & Clinical Warnings

Contraindications:

Known hypersensitivity to thyrotropin or any component of the formulation
Use in animals with known thyroid carcinoma (may stimulate tumor growth)
Use in animals with cardiac disease (may exacerbate arrhythmias or hypertension)

Warnings & Clinical Precautions:

For diagnostic use only; not intended for therapeutic use.
Use with caution in animals with adrenal insufficiency (may precipitate addisonian crisis).
May cause transient hyperthyroidism-like signs in animals with hyperthyroidism.
Reconstituted solution should be used immediately; discard unused portion.
Not for use in food animals unless specifically allowed; withdrawal times must be observed.
Pregnancy: Use only if clearly needed; safety not established.

Adverse Effects & Reactions

Common:

Transient tachycardia
Hypotension
Nausea
Vomiting
Injection site reactions

Serious / Severe:

Anaphylaxis (rare)
Arrhythmias
Thyroid storm in animals with hyperthyroidism

Rare:

Allergic reactions
Seizures (in predisposed animals)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Corticosteroids	May suppress TSH response, leading to false-negative results.	Moderate
Dopamine agonists (e.g., bromocriptine)	May inhibit TSH secretion, affecting test results.	Moderate
Somatostatin analogs (e.g., octreotide)	May suppress TSH secretion, affecting test results.	Moderate
Thyroid hormone supplements (levothyroxine)	May suppress endogenous TSH, but exogenous TSH test is still valid; however, concurrent use may alter baseline T4 levels.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Tachycardia
Hypertension
Hyperthermia
Restlessness
Vomiting
Diarrhea

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Monitor vital signs and cardiac function. Administer fluids and electrolytes as needed. In severe cases, beta-blockers (e.g., propranolol) may be used to control tachycardia. Supportive care until signs resolve.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Thyrotropin is a diagnostic agent and is not typically used in food animals. However, if used, no withdrawal times are established; consult regulatory authorities. For extra-label use in food animals, follow AMDUCA guidelines and observe appropriate withdrawal periods.

Storage, Handling & Regulatory Information

Storage Temperature: Store lyophilized powder at 2-8°C (refrigerated). After reconstitution, use immediately or store at 2-8°C for up to 24 hours.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light. Do not freeze. Discard any unused reconstituted solution after 24 hours.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (Thyrogen) but used extra-label in veterinary medicine.

Extra-Label (Off-Label) Use: In the US, thyrotropin is not FDA-approved for veterinary use; however, it may be used legally under the Animal Medicinal Drug Use Clarification Act (AMDUCA) for extra-label use in animals. For food animals, a withdrawal time must be established by a veterinarian, and the drug must not be used in an extra-label manner that results in violative residues.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Thyrotropin is primarily used as a diagnostic agent for evaluating thyroid function in dogs and horses. The TSH stimulation test is the gold standard for diagnosing hypothyroidism. In dogs, a baseline serum T4 is measured, then thyrotropin is administered IV, and a second T4 sample is taken 6 hours later. A normal response is an increase in T4 above the reference range or at least a doubling of the baseline value. In horses, similar protocols are used. The drug is expensive and may be difficult to obtain; alternatives such as synthetic TSH (thyrotropin alfa) are available but also costly. In cats, the test is rarely performed due to the low incidence of hypothyroidism and the availability of other diagnostic methods. Adverse effects are uncommon but can include transient tachycardia and hypotension. Always monitor animals during the test. For food animals, use is not recommended due to lack of residue data.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)