Total T4 alone is often misleading
· Overdiagnoses hypothyroidism in the presence of non-thyroidal illness or use of certain drugs
· Underdiagnoses hyperthyroidism in cats or from thyroxine overdosage
· Inaccurately assesses adequacy of thyroxine therapy
· Fails to detect autoimmune thyroiditis
· Minimal testing is T4 + free T4, except thyroiditis will still not be detected
· For cats, but not dogs, cTSH can be helpful to identify concurrent hyperthyroidism and renal failure
W. Jean Dodds, DVM