Supplementary Material for: Effect of Hypothyroidism and Hyperthyroidism on Tissue Thyroid Hormone Concentrations in Rat

<b><i>Background and Objective:</i></b> The present study was aimed at determining the effects of experimental hypothyroidism and hyperthyroidism on tissue thyroid hormones by a mass spectrometry-based technique. <b><i>Methods:</i></b> Rats were subjected to propylthiouracil treatment or administration of exogenous triiodothyronine (T₃) or thyroxine (T₄). Tissue T₃ and T₄ were measured by liquid chromatography tandem mass spectrometry in the heart, liver, kidney, visceral and subcutaneous adipose tissue, and brain. <b><i>Results:</i></b> Baseline tissue T₃ and T₄ concentrations ranged from 0.2 to 20 pmol <b>∙</b> g^-1 and from 3 to 125 pmol <b>∙</b> g^-1, respectively, with the highest values in the liver and kidney, and the lowest values in the adipose tissue. The T₃/T₄ ratio (expressed as a percentage) was in the 7-20% range in all tissues except the brain, where it averaged 75%. In hypothyroidism, tissue T₃ was more severely reduced than serum free T₃, averaging 1-6% of the baseline versus 30% of the baseline. The extent of tissue T₃ reduction, expressed as percentage of the baseline, was not homogeneous (p < 0.001), with liver = kidney > brain > heart > adipose tissue. The tissue T₃/T₄ ratio significantly increased in all organs except the kidney, averaging 330% in the brain and 50-90% in the other tissues. By contrast, exogenous T₃ and T₄ administration produced similar increases in serum free T₃ and in tissue T₃, and the relative changes were not significantly different between different tissues. <b><i>Conclusions:</i></b> 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