My patient has low thyroid hormone concentrations, but thyroid stimulating hormone is not elevated. What does this mean?
Depending on the clinical presentation, one of two main possibilities is likely.
The more common explanation is that T4 values often decline in animals with non-thyroidal illness and in animals receiving certain drug therapies (some glucocorticoids or anticonvulsants). Non-thyroidal illness may suppress TSH release from the pituitary via glucocorticoid-mediated inhibition or lower T4 concentrations by altering serum protein binding affinities...
...With this pattern of test results, a trial with T4 supplementation is suggested only if there is a strong clinical presentation consistent with hypothyroidism and if no non-thyroidal illness can be detected...
...While the administration of thyroid hormone to animals which do not have hypothyroidism is generally considered to have minimal risk, large scale studies in human medicine have shown detrimental effects of such treatment in patients that have decreases in serum thyroid hormone concentrations due to non thyroidal illness. (Brent GA and Hershman JM. Thyroxine therapy in patients with severe non-thyroidal illness and low serum thyroxine concentrations. J Clin Endocrinology and Metabolism. 1986, 63:1)