Thyroxine receptors were investigated and measured by radio receptor assay“RRA” in homogenate of Euthyroid, Thyrotoxic and Thyroid carcinoma disease. The results obtained indicated a higher incidence of these receptors in goiter thyroid “i.e. (13.3%) for Euthyroid and(13.64%) for Thyrotoxic” than in tumors tissue “i.e. (10.63%) for Thyroid carcinoma”. Spectroscopic studies in the U.V region were carried out on crude thyroxine receptors “CT4R” and thyroxine hormone “L-T4”. The effects of pH and polarity on the spectra were also studied. Spectrophotometeric pH titration of thyroxine receptors and L-T4 were carried out and revealed that pKa for histidyl residue was (6.6, 7.7, 6.5) for Euthyroid, Thyrotoxic and Thyroid carcinoma respectively, while the pKa value of tyrosine residue was (11) for each one of them.