Hypothyroidism leads to lack of feedback inhibition resulting in increased concentrations of serum TSH. Endogenous serum TSH concentrations are useful in determining if a dog with a low Total T4 is truly hypothyroid, allowing differentiation of true primary hypothyroidism from “euthyroid sick” syndrome in many, but not all, cases. In most cases, serum TSH will eliminate the need for TSH stimulatory tests. Serum TSH measurements are recommended only after an initial Total T4 result is low or low normal. If the hypothyroidism is secondary and a result of lesions within the pituitary gland or higher in the hypothalamus then TSH secretion will be reduced. In these cases serum TSH concentrations will be below the reference range.
Serum TSH measurement can be done on the same serum sample as the initial serum Total T4 measurement. Serum TSH concentrations should not be requested and interpreted alone but need to be interpreted with clinical signs and the concurrent serum Total T4 concentration. Depending on the study, 13-38% of hypothyroid dogs may have serum TSH concentrations within the reference range and serum TSH concentrations may be increased above the reference range in 7.6-18% of normal dogs. Where serum TSH and T4 concentrations are interpreted together there is good diagnostic accuracy. TSH results should be interpreted only in the light of clinical and laboratory evaluation for the presence of non-thyroidal illness.
Normal cTSH: 0.0 – 0.5 ng/mL
Normal cTSH with normal fT4 or TT4 is very likely to reflect the presence of normal thyroid function.
Low fT4 or tT4 with increased cTSH in a patient with the appropriate history and clinical signs supports a diagnosis of primary hypothyroidism.
Other combinations of fT4, tT4 and TSH are hard to interpret and may be seen with hypothyroidism, in the presence of non-thyroidal illness, or reflect the use of medication such as sulphonamides, glucocorticoids or anticonvulsants.
See also “Diagnosis and monitoring of hypothyroidism”.
Canine and feline
Plain (red top) tube or gel serum separator tube