Effects of oestrogen on urinary thyrosine excretion

Disruption by stealth - Interference of endocrine disrupting chemicals on hormonal crosstalk with thyroid axis function in humans and other animals

Thyroid hormones (THs) are important regulators of growth, development, and homeostasis of all vertebrates. There are many environmental contaminants that are known to disrupt TH action, yet their mechanisms are only partially understood. While the effects of Endocrine Disrupting Chemicals (EDCs) are mostly studied as "hormone system silos", the present critical review highlights the complexity of EDCs interfering with TH function through their interactions with other hormonal axes involved in reproduction, stress, and energy metabolism. The impact of EDCs on components that are shared between hormone signaling pathways or intersect between pathways can thus extend beyond the molecular ramifications to cellular, physiological, behavioral, and whole-body consequences for exposed organisms. The comparatively more extensive studies conducted in mammalian models provides encouraging support for expanded investigation and highlight the paucity of data generated in other non-mammalian vertebrate classes. As greater genomics-based resources become available across vertebrate classes, better identification and delineation of EDC effects, modes of action, and identification of effective biomarkers suitable for HPT disruption is possible. EDC-derived effects are likely to cascade into a plurality of physiological effects far more complex than the few variables tested within any research studies. The field should move towards understanding a system of hormonal systems' interactions rather than maintaining hormone system silos.

Directional thyroid hormone distribution via the blood stream to target sites

Thyroid hormones are bound to three major serum transport proteins, thyroxin-binding globulin (TBG), transthyretin (TTR) and human serum albumin (HSA). TBG has the strongest affinity for thyroid hormones, TTR is also found in the cerebrospinal fluid and HSA is the most abundant protein in plasma. Combination defects of either a high affinity TTR or HSA variant do not compensate TBG deficiency, underscoring the dominant role of TBG among the thyroid hormone transport proteins. On the other hand, coexistence of raised affinity TTR and HSA variants causes an augmented hyperthyroxinemia. Variations in thyroid hormone transport proteins may alter thyroid function tests to mimic hypo- or hyperthyroidism. As affected individuals are clinically euthyroid and do not require treatment, identification of thyroid hormone transport protein defects is important to avoid unnecessary diagnostic and therapeutic interventions. Mammals share the multilayered system of thyroid hormone binding proteins with humans. Some of them, especially carnivores, do not express TBG. In dogs, this defect has been shown to be caused by a defective hepatocyte nuclear factor-1 binding site in the TBG promoter, preventing TBG synthesis in the liver. The major endogenous thyroid hormone metabolite 3-iodothyronamine (3-T1AM) exerts marked cryogenic, metabolic, cardiac and central nervous system actions. It is bound to apolipoproteinB-100 (ApoB100), possibly facilitating its cellular uptake via interaction with the low density lipoprotein-receptor. This review summarizes the handling of hydrophobic charged thyroid hormone signaling molecules and their metabolite 3-T1AM in aqueous body fluids and the advantages and limits of their serum distributor proteins.

Effect of phenytoin therapy on thyroid function

Serum total and free fraction of thyroxine and triiodothyronine and urinary losses of unconjugated hormones in normal subjects and in patients treated long-term with therapeutic doses of phenytoin have been measured. Decreases in serum total hormone concentrations with increased free fractions and resultant significant increase in the concentration of free thyroxine but not triiodothyronine were apparent in phenytoin-treated subjects. However, serum free hormone concentrations remained within the euthyroid range. These changes in serum free hormone concentration were reflected by an increased urinary loss of unconjugated thyroxine, but normal excretion of unconjugated triiodothyronine. Phenytoin in therapeutic doses displaces thyroxine and to a lesser extent, triiodothyronine from binding proteins in serum and thus increases peripheral clearance of thyroid hormones.

The assay of urinary thyroid hormones for assessing thyroid function

A competitive protein binding assay for thyroxine (T-4) and a radioimmunoassay for tri-iodothyronine (T-3) were applied to determine the urinary concentrations of these hormones. These measurements were shown to offer excellent diagnostic discrimination between euthyroid, hypothyroid, and hyperthyroid subjects. Urinary T-3 and T-4 levels were not affected by changes in the concentration of thyroxine binding globulin (TBG) or by changes in binding capacity; and therefore, remained normal during pregnancy, oestrogen, and phenytoin therapy. There was no significant circadian variation in urine T-3 excretion. Thus T-3 assays of overnight urine samples could be used in conjunction with dynamic test procedures, such as thyrotrophin (TSH) stimulation test, to enable the rapid assessment of the integrity of the hypothalamic-pituitary-thyroid axis.