From the Research
Lower thyroxine binding globulin (TBG) levels typically don't cause direct symptoms themselves but can affect thyroid function test interpretation, with signs including normal or elevated free T4 and T3 levels despite low total T4 and T3. These signs can misleadingly suggest hypothyroidism if only total hormone levels are measured. Conditions associated with low TBG include androgen excess (as in anabolic steroid use), liver disease, protein-losing conditions like nephrotic syndrome, malnutrition, and genetic TBG deficiency, as noted in studies such as 1. Certain medications like androgens, glucocorticoids, and L-asparaginase can also decrease TBG levels. Since TBG primarily affects the transport of thyroid hormones rather than their activity, patients with low TBG but normal free thyroid hormone levels typically remain euthyroid and don't require treatment for the TBG deficiency itself, as discussed in 2. Diagnosis involves measuring both total and free thyroid hormone levels, along with TBG concentration when available, to avoid misdiagnosis of thyroid dysfunction based solely on total hormone measurements in patients with binding protein abnormalities, a point emphasized in 3. Key factors to consider in the interpretation of thyroid function tests include the potential for interference from heterophile antibodies or changes in TBG levels, as illustrated in case studies such as those presented in 4 and 5. The clinical importance of accurately interpreting thyroid function tests cannot be overstated, as erroneous diagnosis and potentially harmful treatment can be avoided by proving interference or TBG deficiency whenever there is a discrepancy between thyroid function results and the clinical picture, highlighting the need for a comprehensive approach to thyroid function testing that considers TBG levels and other factors that may influence test results.