Hi Matt:
The following is information from labcorp, but you can also find similar information in the Quest Diagnostic's webpage (
www.questdiagnostics.com)
Thyroxine binding globulin
Distinguish between high T4 levels due to hyperthyroidism and due to increased binding by TBG in euthyroid individuals who have normal levels of free hormones; document cases of hereditary deficiency or increase of TBG; work-up of thyroid disease. In patients with low T4, high T3 (uptake) or the reverse, who clinically seem eumetabolic and have normal FTI, measurement of TBG is only occasionally needed. Some such patients may have hereditary anomalies of TBG.
TBG is increased by estrogens, tamoxifen, pregnancy, perphenazine, and in some cases of liver disease, including hepatitis. Decreased TBG is found with some instances of chronic liver disease, nephrosis and systemic disease and with large amounts of glucocorticoids, androgens/anabolic steroids, and acromegaly. Although alterations of TBG are usually resolved by the thyroid profile, TBG must occasionally be directly measured.
Cortisol Binding Globulin
From "The Diagnosis and Differential Diagnosis of Cushings Syndrome and Pseudo-Cushings States", The Endocrine Society 1998
Estrogens increase the cortisol-binding globulin concentration in the circulation; since RIAs measure total cortisol, false positive rates are seen in 50% of women on the oral contraceptive pill (75). It is our routine practice, where possible, and particularly in mild cases, to stop such estrogen-containing drugs and delay investigation for 6 weeks to allow the cortisol-binding globulin to return to baseline. However, this may not be necessary in the case of transdermal estrogens.
75. Tiller JW, Maguire KP, Schweitzer I, Biddle N, Campbell DG, Outch K, Davies BM 1988 The dexamethasone suppression test: a study in a normal population. Psychoneuroendocrinology 13:377384
There are lots of references but these are just some.
The greater this transporter proteins are (TBG and CBG), the less available cortisol and thyroid hormones will be for your tissues.
That's why it is important to measure both free levels of thyroid and cortisol, since the total measurements of these hormones can be altered by estrogens and other things.
Regards,
Roberto