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

The present study was aimed at determining the effects of experimental hypothyroidism and hyperthyroidism on tissue thyroid hormones by a mass spectrometry-based technique.

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.

Baseline tissue T3 and T4 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 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.

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.