Veterinary Drug Handbook (VDH) is the reference veterinarians turn to when they want an independent source of information on the drugs that are used in veterinary medicine today.


Dogs: Dog

For hypothyroidism:
a) Use a trade name product initially, with a starting dosage of 20 micrograms/kg bodyweight PO q12h. Some dogs may only require once a day dosing, but initially, all dogsshould receive twice daily dosing. After clinical symptoms have resolved, once dailydosing may be attempted. Dogs who have concomitant cardiac problems should receive5 micrograms/kg twice daily; may increase dosage gradually over 3-4 weeks. (Nelson1989b)
b) Initiate treatment at 22 micrograms/kg PO twice daily (0.1 mg/10 lbs body weight bid);reevaluate dosage after monitoring clinical response and serum levels after 4-8 weeks.
If clinical response is satisfactory and T4 is elevated (≥ 60 nmol/L) may reduce dosageto 22 micrograms/kg once daily. If clinical response is not satisfactory, either reevaluatethe need for T4 supplementaion or increase the dose. Daily dosage of 20 - 40micrograms/day appears to be adequate for most dogs. (Refsal and Nachreiner 1995)
c) 0.02 - 0.04 mg/kg daily, or perhaps more logically: 0.5 mg/m2 daily. Many dogs mayonly need once daily dosing, but some will require twice daily doses. In patients withhypoadrenocorticism, cardiac disease, diabetes, or those who are aged, it is recommended to start low (0.1 mg/m2 bid), and increase dosage by 20-25% increments over4-8 weeks. In hypoadrenal dogs, start glucocorticoid supplementation before thyroidreplacement. All patients should have therapy assessed after 6-12 weeks using clinicalor laboratory criteria. (Ferguson 1986)

Cats: Cat

For hypothyroidism:
a) Initially, 10 - 20 micrograms/kg per day; adjust dosage on basis of clinical response andpost-dose serum thyroxine level. (Peterson and Randolph 1989)
b) Initially, 0.05 - 0.1 mg once daily. Wait a minimum of 4-6 weeks to assess cat's clinicalresponse to treatment. Then obtain a serum T4 level prior to, and 6-8 hours after, dosing. Increase or decrease dose and/or dosing frequency after reviewing these valuesand clinical response. If levothyroxine is ineffective, may try liothyronine. (Feldman and Nelson 1987d)

Horses: Horse

For hypothyroidism:
a) 10 mg in 70 ml of corn syrup once daily. Monitor T4 levels one week after initiation oftherapy. Obtain one blood sample just before administration and on sample 2-3 hoursafter dosing. (Chen and Li 1987)

Birds: Bird

For hypothyroidism:
a) One 0.1 mg tablet in 30 ml - 120 ml of water daily; stir water and offer for 15 minutesand remove. Use high dose for budgerigars and low dose for water drinkers. Used forrespiratory clicking, vomiting in budgerigars and thyroid responsive problems. (Clubb1986)

Reptiles: Reptile

a) For hypthyroidism in tortoises: 0.02 mg/kg PO every other day. (Gauvin 1993)
Monitoring Parameters -
1) Serum thyroid hormone concentrations (T4/T3). Monitoring before therapy is begun canhelp confirm diagnosis. After therapy is started wait at least 5-10 days before measuring
T4, one month may be better, especially if dosage is ineffective or symptoms ofthyrotoxicosis develop. Serum levels should be drawn before the dose and 6-8 hoursafter. Dosage should be reduced if serum thyroxine levels exceed 100 ng/ml or symptomsof thyrotoxicosis develop.
Client Information - Clients should be instructed in the importance of compliance with therapy asprescribed. Also, review the symptoms that can be seen with too much thyroid supplementation (see
Overdosage section above).
Dosage Forms/Preparations/FDA Approval Status - All levothyroxine products require a prescription. There have been bioavailability differences between products reported. It is recommendedto use a reputable product and not to change brands indiscriminately.
Veterinary-Approved Products -
Levothyroxine Sodium Tablets 0.1 mg, 0.2 mg, 0.3 mg, 0.4 mg, 0.5 mg, 0.6 mg, 0.7 mg, 0.8 mg;
Soloxine® (Daniels), Thyro-Tabs ® (Vet-A-Mix); Thyrozine Tablets® (Anthony) (Rx)
Approved for use in dogs.
Levothyroxine Sodium Tablets Chewable (Veterinary) 0.2 mg, 0.5 mg, 0.8 mg; Thyro-Form®(Vet-A-Mix) (Rx) ¯ Approved for use in dogs.
Levothyroxine Sodium Tablets Chewable (Veterinary) 0.1 mg, 0.2 mg, 0.3 mg, 0.4 mg, 0.5 mg, 0.6 mg, 0.7 mg, 0.8 mg; HESKA Chewable Thyroid Supplement for Dogs (Heska); Approvedfor use in dogs (Rx)
Levothyroxine Sodium Powder (Veterinary) 0.22% (1 gram of T4 in 454 grams of powder):
One level teaspoonful contains 12 mg of T4. Available in 1 lb. and 10 lb. containers.; Thyro-L® (Vet-A-Mix) (Rx) ¯ Approved for use in horses.
Human-Approved Products -
Levothyroxine Sodium Tablets 0.025 mg, 0.05 mg, 0.075 mg, 0.088 mg, 0.1 mg, 0.112 mg, 0.125 mg, 0.137 mg, 0.15 mg, 0.175 mg, 0.2 mg, 0.3 mg; Synthroid® (Knoll), Levothroid®(Forest); Levo-T® (Lederle); Levoxyl® (Daniels); Eltroxin® (Roberts); generic, (Rx)
Levothyroxine Powder for Injection 200 micrograms per vial, 500 micrograms/vial in 6 ml and10 ml vials; Synthroid® (Knoll), Levothroid® (Forest); Levoxine® (Daniels), generic (Rx)

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