Thyroid binding globulin

Thyroid Hormones Are Transported by Thyroid-Binding Globulin... 

Hemopexin (binds heme) Retinol-binding protein (binds retinol) Sex hormone-binding globulin (binds testosterone, estradiol) Thyroid-binding globulin (binds T4, T3) Transferrin (transport iron)... 

Thyroid hormone is liberated into the bloodstream by the process of proteolysis within thyroid cells. T4 and T3 are transported in the bloodstream by three proteins thyroid-binding globulin, thyroid-binding prealbumin, and albumin. Only the unbound (free) thyroid hormone is able to diffuse into the cell, elicit a biologic effect, and regulate thyroid-stimulating hormone (TSH) secretion from the pituitary. 

In thyrotoxic Graves disease, there is an increase in the overall hormone production rate with a disproportionate increase in T3 relative to T4 (Table 20-1). Saturation of thyroid-binding globulin is increased due to the elevated levels of serum T4 and T3, which is reflected in an elevated T3 resin uptake. As a result, the concentrations of free T4, free T3, and the free T4 and T3 indices are increased to an even greater extent than are the measured serum total T4 and T3 concentrations. The TSH level is undetectable due to negative feedback by elevated levels of thyroid hormone at the pituitary. In... 

TBG, thyroid-binding globulin TSH, thyroid-stimulating hormone T3, triiodothyronine Lp thyroxine. 

Oral contraceptives have their most significant effect on endocrine parameters. Blood cortisol, thyroxine, protein-bound iodine, T3 uptake, and urinary free cortisol are elevated. Urinary 17,21-dihydroxy steroids, 17-ketosteroids, and estrogens are decreased. There is no effect on urinary catecholamines or VMA (Table 10) (LIO). The effect of thyroid functions tests is due to the administered hormone stimulating an increase in the production of thyroid-binding globulin which in turn binds 1-thyroxine. The lowering of free thyroxine stimulates the anterior pituitary to produce thyrotropin, which in turn stimulates the thyroid to produce more thyroxine. Since the additional thyroxine is bound to the extra protein, there is an equilibrium and the patient remains clinically euthyroid, but the protein-bound iodine and the thyroxine are elevated. 

Hypothyroidism Estrogen administration leads to increased thyroid-binding globulin (TBG) levels. 

Cheek AO, Kow K, Chen J, McLachlan JA (1999) Potential mechanisms of action of thyroid disruption in humans Interaction of organochlorine compounds with thyroid receptor, transthyretin, and thyroid-binding globulin. Environ Health Perspect, 107(4) ... 

An absence of R-type binding protein has been reported in two adult siblings by Carmel and Herbert (C20). R-Type protein was virtually absent from their leukocytes and saliva, and as was expected they had very low levels of serum vitamin B12. The absence of the protein did not appear to have any adverse effects. Other members of this family have also been found to have an absence of, or very low levels of, R-protein (H26). There was no general deficiency of plasma glycoproteins in these patients and the amounts of thyroid binding globulin, thyroxine, ceruloplasmin, and transferrin were all normal. 

H4. Harvey, R. F., Semm thyroxine and thyroid binding globulin in seriously ill patients. Lancet i, 208-212 (1971). 

Through competitive binding to thyroid-binding globulin, salicylates in high concentrations can displace thyroxine and triiodothyronine, thus interfering with the results of diagnostic thyroid function tests (118). 

The health effect of endocrine disruptors is further complicated by the fact that an endocrine disrupter or a family of endocrine disruptors may have multiple mechanisms of actions. For example, PCBs may mimic estrogen, prevent binding of thyroid hormone to thyroid binding globulin, and accelerate the metabolism and excretion of several steroid hormones. 

Greene H, Hwang D, Hutchins J, Johnson A. Techni-con thyroxine and thyroid binding globulin Two fuHy automated immunoassays for the determination of total thyroxine and TBG in human serum, Clin Chem 1990 36 1082. 

McCormack G> Faix J. Evaluation of chemiluminescent enzyme immunoassay for thyroid binding globulin (TBG) and comparison of T4/TBG ratio with free thyroxin index. Clin Chem 1990 36 1149. 

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