If there is no change in T3 and the baseline T3 and T4 concentrations are not clearly elevated, we cannot make a correct interpretation of the test. We need to document an increase in 2 to 3 hour post pill T3 to demonstrate that the medication (Cytomel, exogenous T3) was adequately administered and absorbed. The first step in interpreting a T3 suppression test is to compare the pre- and post- pill T3 concentration. To continue investigating the possibility of hyperthyroidism, consider doing one of the following: 1) repeat the profile after a few weeks, 2) perform a T3 suppression test. In most of these cases, however, the thyroid hormone concentration is within the top half, or more often the top quarter, of the reference range.

Therefore, the finding of normal thyroid hormones in a cat with appropriate clinical findings does not rule out hyperthyroidism. This phenomenon is termed 'occult hyperthyroidism.' A number of possible explanations exist for this phenomenon including the presence of thyroid pathology which is not yet sufficiently severe to be easily documented (mild disease) the presence of a concurrent non-thyroidal illness and day-to-day or week-to-week fluctuations in thyroid hormone concentrations in hyperthyroid cats which sometimes dip into the reference range. However, there are a small proportion of hyperthyroid cats which have normal or only slightly elevated thyroid hormone concentrations. Most hyperthyroid cats have high serum concentrations of thyroid hormones. Hyperthyroidism is rarely diagnosed in cats less than 6-7years of age. Remember signalment is important in considering a diagnosis of hyperthyroidism. In a cat with appropriate clinical signs of hyperthyroidism, a first step would be to perform a feline thyroid profile.

Euthyroid sick syndrome mayo clinic