Effect of Hypothyroidism and Hyperthyroidism on Tissue Thyroid Hormone Concentrations in Rat

Background and Objective: The present study was aimed at determining the effects of experimental hypothyroidism and hyperthyroidism on tissue thyroid hormones by a mass spectrometry-based technique. Methods: Rats were subjected to propylthiouracil treatment or administration of exogenous triiodothyronine (T 3 ) or thyroxine (T 4 ). Tissue T 3 and T 4 were measured by liquid chromatography tandem mass spectrometry in the heart, liver, kidney, visceral and subcutaneous adipose tissue, and brain. Results: Baseline tissue T 3 and T 4 concentrations ranged from 0.2 to 20 pmol ∙ g –1 and from 3 to 125 pmol ∙ g –1 , respectively, with the highest values in the liver and kidney, and the lowest values in the adipose tissue. The T 3 /T 4 ratio (expressed as a percentage) was in the 7–20% range in all tissues except the brain, where it averaged 75%. In hypothyroidism, tissue T 3 was more severely reduced than serum free T 3 , averaging 1–6% of the baseline versus 30% of the baseline. The extent of tissue T 3 reduction, expressed as percentage of the baseline, was not homogeneous (p < 0.001), with liver = kidney > brain > heart > adipose tissue. The tissue T 3 /T 4 ratio significantly increased in all organs except the kidney, averaging 330% in the brain and 50–90% in the other tissues. By contrast, exogenous T 3 and T 4 administration produced similar increases in serum free T 3 and in tissue T 3 , and the relative changes were not significantly different between different tissues. Conclusions: While the response to increased thyroid hormones availability was similar in all tissues, decreased thyroid hormone availability induced compensatory responses, leading to a significant mismatch between changes in serum and in specific tissues.