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Information leaflet

                                                   Irish Medicines Board

                               Summary of Product Characteristics

 Forthyron 800 microgram tablets for dogs.


 One tablet contains:
 800 microgram levothyroxine sodium per tablet equivalent to 778 microgram levothyroxine.
 For a full list of excipients, see section 6.1.


 White to off white round tablets, scored on one side.
 The tablets may be divided into halves or quarters.


 4.1 Target Species


 4.2 Indications for use, specifying the target species

 For the treatment of hypothyroidism in dogs.

 4.3 Contraindications

 Do not use in dogs suffering from uncorrected adrenal insufficiency.
 Do not use in cases of known hypersensitivity to levothyroxine sodium

 4.4 Special warnings for each target species

 The diagnosis hypothyroidism should be confirmed with appropriate tests.

Date Printed 25/11/2011                               CRN 7008718                                         page number: 1
                                                   Irish Medicines Board

 4.5 Special precautions for use

 Special precautions for use in animals
 A sudden increase in demand for oxygen delivery to peripheral tissues, plus the chronotropic effects of levothyroxine
 sodium, may place undue stress on a poorly functioning heart, causing decompensation and signs of congestive heart
 failure. Hypothyroid dogs suffering from hypoadrenocorticism have a decreased ability to metabolise levothyroxine
 sodium and therefore an increased risk of thyrotoxicosis. Dogs with concurrent hypoadrenocorticism and
 hypothyroidism should be stabilised with glucocorticoid and mineralocorticoid treatment prior to treatment with
 levothyroxine sodium to avoid precipitating a hypoadrenocortical crisis. After this, thyroid tests should be repeated,
 then gradual introduction of levothyroxine therapy, starting with 25% of the normal dose, increasing by 25%
 increments every fortnight until optimal stabilisation is achieved is recommended. Gradual introduction of therapy is
 also recommended for dogs with other concurrent illnesses; particularly in dogs with cardiac disease, diabetes mellitus
 and renal or hepatic dysfunction.

 Special precautions to be taken by the person administering the medicinal product to animals
 Wash hands after administering the tablets. Pregnant women should handle the product with caution. In the case of
 accidental ingestion, seek medical advice immediately and show the package leaflet or the label to the physician. Note:
 this product contains a hign concentration of L-thyroxine sodium and may present a risk to humans, in particular
 children, if ingested.

 4.6 Adverse reactions (frequency and seriousness)

 Restoration of physical activity may unmask or intensify other problems, such as osteoarthrosis. Adverse effects of
 thyroid hormones are generally associated with excessive dosage and correspond to the symptoms of hyperthyroidism.
 See also section 4.10.

 4.7 Use during pregnancy, lactation or lay

 The safety of the veterinary medicinal product has not been established in pregnant or lactating bitches. However,
 levothyroxine is an endogenous substance and thyroid hormones are essential for the developing foetus, especially
 during the first period of gestation. Hypothyroidism during pregnancy may result in major complications such as foetal
 death and a poor perinatal outcome. Maintenance dose of levothyroxine sodium may need adjustment during
 pregnancy. Pregnant bitches should therefore be monitored on a regular base from conception until several weeks after

 4.8 Interaction with other medicinal products and other forms of interaction

 A variety of drugs may impair plasma or tissue binding of the thyroid hormones or alter thyroid hormone metabolism
 (eg. barbiturates, antacids, anabolic steroids, diazepam, furosemide, mitotane, phenylbutazone, phenytoin, propranolol,
 large doses of salicylates, and sulphonamides.). When treating dogs that are receiving concurrent medication the
 properties of these drugs should be taken into consideration.
 Estrogens may increase thyroid requirements.
 Ketamine may cause tachycardia and hypertension when used in patients receiving thyroid hormones. The effect of
 catecholamines and sympaticomimetics is increased by levothyroxine.
 An increase in the dosage of digitalis may be necessary in a patient that had previously compensated congestive heart
 failure and that is placed on thyroid hormone supplementation.
 Following treatment of hypothyroidism in dogs with concurrent diabetes, careful monitoring of diabetic control is

 Most dogs on chronic high- dose, daily glucocorticoid therapy will have very low or undetectable serum T4
 concentrations, as well as subnormal T3 values.

Date Printed 25/11/2011                               CRN 7008718                                         page number: 2
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