Do not use in dogs suffering from thyrotoxicosis or uncorrected adrenal insufficiency. Administer with caution to animals with clinically significant heart disease, or other complications for which a sharply increased metabolic rate might prove hazardous.

Restoration of physical activity may unmask or intensify other problems, such as arthritis.

Adverse effects of thyroid hormones are generally associated with excessive dosage and correspond to the symptoms of hyperthyroidism.

A sudden increase in demand for oxygen delivery to peripheral tissues, plus the chronotropic effects of T4, 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 T4 and therefore an increased risk of thyrotoxicosis. For these dogs, gradual introduction of levothryroxine therapy starting with 25% normal dose, increasing by 25% increments every fortnight until optimal stabilisation is achieved.

Adjustments 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. Prednisone increases T4 binding to serum carrier proteins. This may result in lower T4 transfer rates from serum to extravascular sites and increased extravascular T4 binding. Insulin requirements are decreased in the hypothyroid patient with diabetes.

Overdoses of three to six times label recommended starting dose for four consecutive weeks resulted in no significant clinical signs that could be attributed to treatment. However, following chronic over supplementation, overdose will eventually lead to thyrotoxicosis, manifested by: weight loss without anorexia, polydispia, polyuria, panting and probably tachycardia, and nervousness. The presence of these signs should result in evaluation of T4 serum concentration to confirm the diagnosis, and immediate discontinuation of the supplement.

Forthyron® may be used during pregnancy or lactation according to a risk:benefit assessment. The dosage may have to be increased if symptoms of hypothyroidism return.

For animal treatment only. Keep out of reach and sight of children.