Thyroid hormone-binding globulin

Prolonged therapy with methadone causes increases in serum thyroid hormone-binding globulin, triiodothyronine, and thyroxine, as well as albumin, globulin, and prolactin, and these must be monitored (SEDA-15, 71 SEDA-17, 81). 

Interferences No clinically significant effect is exercised on the assay by haemolytic (haemoglobin up to 12 g/1), lipaemic (triglycerides up to 1200 mg/dl) and icteric (bilirubin up to 3.5 mg/dl) samples. Neither albumin nor thyroid hormone binding globulin (TBG) interfere with the assay. Phenylbutazone (up to 0.3 mg/1), phenytoin (up to 30 mg/1) and salicylate (up to 400 mg/1) will not interfere. 

Lipophilic hormones that use intracellular gene-specific transcription factors include the steroid hormones, thyroid hormone, retinoic acid (active form of vitamin A), and vitamin D (Fig. 11.8). Because these compounds are water-insoluble, they are transported in the blood bound to serum albumin, which has a hydrophobic binding pocket, or to a more specific transport protein, such as steroid hormonebinding globulin (SHBG) and thyroid hormone-binding globulin (TBG). The intracellular receptors for these hormones are structurally similar and are referred to as the steroid hormone/thyroid hormone superfamily of receptors. 

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

Determination of Thyroxine-Binding Globulin AND Other Thyroid Hormone-Binding Proteins... 

Thyroid-birMring globulin (TBG) Thyroid hormone binding Investigation of thyroid disease... 

Larsson, M., T. Pettersson, and A. Carlstrom. 1985. Thyroid hormone binding in serum of 15 vertebrate species Isolation of thyroxine-binding globulin and prealbumin analogs. General Comparative Endocrinology 58 360-375. 

Thyroid hormone binds to its appropriate receptor in the cytoplasm and is translocated to the nucleus, where it induces the synthesis of specific proteins (Sterling, 1979 Oppenheimer and Surks, 1975). The thyroid hormone receptor does not enter into the nucleus instead, the thyroid hormone (T3) enters and binds to a separate receptor in the chromatin (Sterling, 1979). In addition, T3 binds to a specific receptor in mitochondria which is separate from the cytoplasmic thyroid hormone receptor (Sterling, 1979). T3 stimulates the appearance of the protein Q 2u-globulin, which also requires testosterone for its synthesis (Kurtz et al., 1976 Chan et al., 1978). Furthermore, cells cultured from a rat pituitary tumor are stimulated by T3 to produce the mRNA for growth hormone (Shapiro et al., 1978). There are a number of problems in ascribing all of the actions of T3 to induction of protein synthesis by the action of T3 at the nuclear level. Similarly, it is not possible to account for all of the actions of T3 at the mitochondrial level. 

Thyroxine (3, 5, 3,5-L-teraiodothyronine, T4) is a thyroid hormone, which is transformed in peripheral tissues by the enzyme 5 -monodeiodinase to triiodothyronine. T4 is 3-8 times less active than triiodothyronine. T4 circulates in plasma bound to plasma proteins (T4-binding globulin, T4-binding prealbumin and albumin). It is effective in its free non-protein-bound form, which accounts for less than 1%. Its half-life is about 190 h. 

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

The thyroid hormones are lipophilic and relatively insoluble in the plasma. Therefore, they are transported throughout the circulation bound to plasma proteins such as thyroxine-binding globulin (75%) and albumins (25%). Approximately 99.96% of circulating thyroxine is protein bound. Bound hormone is not available to cause any physiological effects however, it is in equilibrium with the remaining 0.04% that is unbound. This free form of the hormone is able to bind to receptors on target tissues and cause its effects. Thyroid hormone has many metabolic effects in the body ... 

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. 

Thyroid function tests - Decreased levels of thyroxine-binding globulin, resulting in decreased total T4 serum levels and increased resin uptake of T3 and T4. Free thyroid hormone levels remain unchanged, and there is no... 

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