Thyrotropin stimulation test—new perspective on value of monitoring triiodothyronine

Abstract

SUMMARY Thyrotropin (thyroid stimulating hormone; tsh ) stimulus to thyroid cells of horses and dogs resulted in increased serum triiodothyronine (T 3 ) concentrations that were detected earlier than those of thyroxine (T 4 ). Doubling of the base-line T 3 values in horses was detected 0.5 hours after injection of 5IU of tsh iv , with peak response of 5 times base-line value detected 2 hours after injection. Doubling of T 4 values in horses was noticed between 2 and 3 hours, with the peak response of 2.4 times base-line value at 4 hours after injection of tsh . Doubling of base-line T 3 values in dogs in response to 0.2 IU tsh /kg of body weight ( iv -5 IU maximum dose) was noticed at 1 hour, whereas T 4 response doubled between 1.5 and 2 hours. Peak release of T 3 and T 4 in response to tsh in dogs had not developed by 4 hours; however, the percentage increase over base-line values was greater for T 3 than T 4 at early sampling time points, and this response has resulted in an increased T 3 /T 4 ratio in hypothyroid dogs. Thus, in both dogs and horses, these studies indicated that T 3 response to tsh could be used as a measure of thyroid function at earlier time intervals after tsh administration than one measures T 4 response.