Serum thyroid hormone levels, change

Children s Susceptibility. Neurodevelopmental delays and postnatal changes in serum thyroid hormone levels have been observed in animals following exposure of their mothers to chlorine dioxide or chlorite during gestation and/or lactation (Carlton and Smith 1985 Carlton et al. 1987 Gill et al. 2000 Mobley et al. 1990 Orme et al. 1985 Taylor and Pfohl 1985 Toth et al. 1990). It is not known whether age-related differences in toxicokinetic parameters exist for chlorine dioxide or chlorite. Additional studies should be designed to further examine neurodevelopmental toxicity and underlying mechanisms. 

Silvestri E, A. Lombardi, P. de Lange, L. Schiavo, A. Lanni, F. Goglia, T.J. Visser and M. Moreno, 2008. Age-related changes in renal and hepatic cellular mechanisms associated with variations in rat serum thyroid hormone levels. Am J Physiol Endocrinol Metab. 294, E1160-1168. 

Many of the adverse effects of lithium can be ascribed to the action of lithium on adenylate cyclase, the key enz)nne that links many hormones and neurotransmitters with their intracellular actions. Thus antidiuretic hormone and thyroid-stimulating-hormone-sensitive adenylate cyclases are inhibited by therapeutic concentrations of the drug, which frequently leads to enhanced diuresis, h)rpoth)n oidism and even goitre. Aldosterone synthesis is increased following chronic lithium treatment and is probably a secondary consequence of the enhanced diuresis caused by the inhibition of antidiuretic-hormone-sensitive adenylate cyclase in the kidney. There is also evidence that chronic lithium treatment causes an increase in serum parathyroid hormone levels and, with this, a rise in calcium and magnesium concentrations. A decrease in plasma phosphate and in bone mineralization can also be attributed to the effects of the drug on parathyroid activity. Whether these changes are of any clinical consequence is unclear. 

Treatment-related altered serum th5Toid hormone levels indicate that chlorine dioxide and chlorite may exert toxic effects that are mediated through the neuroendocrine axis. Changes in thyroid hormones have been reported in laboratory animals that were either directly exposed to chlorine dioxide (repeated doses as low as 9 mg/kg/day), or exposed to chlorine dioxide or chlorite via their mothers (maternal doses of chlorine dioxide and chlorite as low as 13 and 9 mg/kg/day, respectively) during pre- and postpartum development (Bercz et al. 1982 Carlton and Smith 1985 Carlton et al. 1987, 1991 Mobley et al. 1990 Orme et al. 1985). 

Thyroid effects that have mainly included reduced serum T4 hormone levels and follicular cell hyperplasia were consistently observed in rats and mice orally exposed to PBDEs. Accompanying changes in serum TSH levels were not found and the depression of serum T4 is likely related to hepatic enzyme induction. Acute duration studies showed decreases in serum T4 in rats exposed to >10 mg/kg/day octaBDE or >30 mg/kg/day pentaBDE for 4 days and in rats and mice exposed to >18 mg/kg/day pentaBDE for 14 days. Effects observed in intermediate-duration studies include thyroid hyperplasia in rats exposed to >8 mg/kg/day octaBDE for 30 days and reduced serum T4 in rats exposed to >10 mg/kg/day pentaBDE for 90 days. Exposure to pentaBDE on gestation day 6 through postnatal day 21 caused serum T4... 

The mechanisms of disruption of thyroid function have not been established. Direct interaction of CMs with thyroid hormone receptors or serum thyroid binding proteins seems unlikely but has not been investigated. The adverse histophysiological alterations in the thyroid and change.s in serum thyroid homione levels may be a result of CMs directly interfering with the iodide transporter or... 

Serum thyroid-stimulating hormone (TSH) and thyroid hormone levels should be measured in any patient with a goiter to determine the hormonal status. TSH concentrations increase with age, but the levels remain within the normal range in the healthy population throughout life up to 100 years of age (Canaris et al., 2000 Mariotti et al, 1995). See Chapter 106 by Diez and Iglesias on Hypothyroidism in the Middle Aged and Elderly Clinical Aspects for details of thyroid hormone changes in the elderly. Serum calcitonin levels are not needed unless there is a family history of medullary thyroid cancer or multiple endocrine neoplasia (MEN) type 2. 

Interesting changes have been found in thyroid hormone levels in the neonatal period. It has been known for many years that serum T4 levels are normally higher in infancy than at any other time. Such elevations could well obscure a diminution in thyroid hormone levels associated with hypothyroidism, particularly partial hypothyroidism associated, for example, with dyshormonogenesis. Early detection of hypothyroidism is essential as treatment is simple and the consequences of delay in treatment are severe and to some extent, irreversible. 

One explanation for the changed response to digitalis with carbimazole is that there is a direct and altered response of the heart due to the raised or lowered thyroid hormone levels. Another is that changes in glomerular filtration rate associated with hypo- or hyperthyroidism result in increased or decreased serum digoxin, respectively. Why carbimazole reduced serum digoxin in healthy subjects (normal thyroid status) is not known. 

However, chronic administration of moderate to large doses of iodide decreases serum thyroid hormone concentrations and increases serum TSH levels although changes are within the normal range. Furthermore, chronic administration of iodide increases significantly the size of thyroid glands in normal subjects. 

Available intermediate- and chronic-duration oral studies in animals indicate that the thyroid and liver are the main systemic targets of PBDE toxicity as shown by effects mainly including enlargement and histological alterations in both organs and changes in serum levels of thyroid hormones. Several acute-duration studies of pentaBDE suggest that immunosuppression may also be an important health end point. Very little information is available on potential neurotoxic effects of PBDEs, mainly the results of three... 

It has been suggested that thyroid status is a possible major regulator of metabolic rate. Herlihy et al. [38] found that CR reduces the 24-hour mean serum triiodothyronine (T3) level, but not the thyroxine (T4) level. CR appears to have this action because it attenuates the circadian amplitude in the concentrations of both hormones. However, Snyder et al. [39] found no effect of CR on either T4 or T3 in young rats and no consistent effect of CR on age-related changes in the concentrations of these two hormones. 

They are now many modifications of different sensitivity and limit of detection (Utiger 1979, Spencer 2004). In the endocrine safety pharmacology in rats and dogs, the TRH test injection with measurement of the serum TSH response is an established tool for assessment of changes at the level of thyroid hormone secretion, with consecutive modification of the pituitary TSH and prolactin response. 

This test is valuable as single-dose uptake test, and maybe performed when changes in the serum concentrations of T3 and T4 if questionable relevance have been found, without that corresponding increase or decrease in the serum TSH concentrations. It may help to differentiate changes which occur or predominantly at the level of thyroid hormone synthesis. 

Little is known about the synthesis of TSH. Direct measurement of serum TSH by radioimmunoassay techniques (R6, Ul) or in vitro or in vivo bioassays (A3, K3) showed that TSH in blood decreased when thyroid hormone was given to hypothyroid persons. Free thyroxine levels were more directly related to the changes in TSH concentration (R6). The half-life of TSH in blood as measured by the use of I-labeled h-TSH is about 30-60 minutes (02, 03). 

Administration of moderately elevated doses of iodide to rats, mice, or humans has immediate effects on thyroid function, independently of later possible effects through changes in thyroid hormone serum levels. These effects... 

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