Mouse Anti-Triiodothyronine monoclonal antibody, clone 21661

Triiodothyronine (T3) is the physiologically active thyroid hormone (TH), so fluctuations in tissue levels of T3 may reflect the role of T3 in any given tissue, all else being equal. A better understanding of changes in tissue T3 content can assist clinicians in understanding the disease state of their patients. Examples include the recovery of TH and T3 function during the treatment of hypothyroid patients, and how the role of TH is affected in patients who restrict their caloric intake or have non-thyroidal illness syndrome (NTIS). However, existing techniques for studying tissue T3 levels are usually invasive and have little clinical applicability, so T3 effects are generally inferred from serum T3 levels.

Figure 1. TRH-TSH-T3 feedback loop
(Source: Kim HY, et al. 2013)

Improvements in the sensitivity and specificity of thyroid testing have led to the detection and treatment of thyroid disease. Initially thyroid function was assessed by basal metabolic rate and bioassays, but later laboratory tests were added to clinical thyroid diagnosis, mainly by measuring TH by measuring protein-bound iodine, a method that indirectly estimates total T4 (TT4) concentration. Because abnormal TH-binding proteins can interfere with TH measurement and make TT4 assessment inaccurate, indirect TBG assessment methods were subsequently developed to provide indirect estimates of free T4. Current assays using TH-binding ratios use primarily autoimmune preparations and non-isotopic signals to assess available TBG-binding sites relative to a "normal" reference value. Depending on the method, the reference value may be set at 1.00 or 40%.

99.7% of T3 is bound to proteins, and direct measurement of free T3 (fT3) is theoretically a better indicator of thyroid function than total T3 (TT3). However, T3 is less concentrated in the circulation than T4 and has a weaker binding affinity for carrier proteins in serum than T4. fT3 measurements are more susceptible to interference from free fatty acids and drugs in the circulation. As a result, fT3 immunoassays are less accurate and reproducible than fT4. In clinical applications, T3 concentrations in patients with hypothyroidism are generally maintained within the reference interval, so T3 testing in patients with suspected hypothyroidism or elevated thyrotropin is of limited clinical value. Circulating T3 is elevated before T4 in patients with hyperthyroidism, so T3 analysis may provide clinically relevant information in patients with suppressed TSH. Deiodinase activity in patients with NTIS may be altered with the disease, resulting in a conversion of T4 to T3, a condition that may cause patients to develop low T3 concentrations.

Anti-T3 monoclonal antibody