Historia de las pruebas de función tiroidea / History of thyroid function tests

El desarrollo de las pruebas de función tiroidea no ha sido fácil, con múltiples retos para mejorar algunas características que son insatisfactorias, incluso en la actualidad. En 1960 se logró la medición de tiroxina total (T4 total), y aunque fue un gran avance, los investigadores sabían que era insuficiente para una evaluación precisa de la función tiroidea. Uno de los problemas importantes radica en que existen diferencias marcadas interindividuales en la composición y en las cantidades de las proteínas de transporte de la T4 y la triyodotironina (T3). Por lo tanto, los depósitos de T4 y T3 son muy diferentes a los valores de T4 libre (T4L) y T3 libre (T3L). Por ejemplo, la mujer embarazada tiene el doble de globulina fijadora de tiroxina (TBG) y tres cuartas partes de la cantidad de albúmina que tenía cuando no estaba embarazada. También se pierde transtiretina y albúmina en enfermedades graves o con traumas, como quemaduras o sepsis. Entre 1963 y 1965 se desarrolló una prueba para tratar de obtener una estimación de la T4L, con el método de absorción de la hormona tiroidea a partir de la T4 total. Sin embargo, este análisis no funcionó correctamente, especialmente teniendo en cuenta la variabilidad en la TBG

Environmental and hormonal control of vernal migration in redheaded bunting (Emberiza bruniceps).

This paper reviews the factors and mechanisms which result in the development of the metabolic state characteristic of migration with special reference to a palaeotropic migrant the redheaded bunting, Emberiza bruniceps. Changes in climatic conditions and food supply act as proximate triggers of migratory behaviour in partial migrants. Typical migrants like buntings use daylength as a cue but the exact mechanism of how photoperiodic information is translated in terms of migratory events is still not known. Almost entirely the photoperiodic effects have been explained on the basis of the involvement of hypothalamo/hypophyseal system. We feel mechanism(s) other than those acting through neuroendocrine system may be equally important. Furthermore the role of temperature has not been adequately explored so far. Our observations indicate the possibility that redheaded buntings might integrate the information received from photoperiod with environmental temperature (and other factors?) resulting in the development of migratory state. The physiological control of avian migration is much less understood. Majority of papers have centered around the ‘gonadal hypothesis’ of Rowan supporting or contradicting it without providing conclusive evidence. Pituitary prolactin has also been shown to be implicated although the mechanism of action is only speculative. Conclusive evidence for the involvement of thyroid hormones (thyroxine, T4; triiodothyronine, T3) in the physiological timing of migration has been produced attributing independent roles to T4 and T3. It is suggested that seasonal variation in peripheral conversion of T4 to T3 could serve as an effective strategy to render available the required thyroid hormones T4 and/or T3 during different phases of the year thus accounting for the metabolic switch over from T4-dependent moult to T3-dependent migratory fat deposition and zugunruhe and also ensuring preclusion of simultaneous occurrence of these mutually incompatible events. Considering that the number of environmental and physiological factors influence this mechanism and considering that thyroid hormone molecule has been put to a wide range of usage during the course of evolution the mechanism(s) of peripheral conversion of T4-T3 may assume great flexibility and have selective value-especially in migration which is known to have evolved several times in diverse avian families. The attractiveness of this hypothesis lies in the fact that it has potential to explain the both physiological development of the metabolic state of migration and at the same time the physiological timing of migration not only with respect to the cycle of environment but also with respect to other conflicting seasonal events (moult and reproduction).

Circulating levels of T4, T3 and TSH in newborns during the first 7 days of life.

Serum total thyroxine (TT4) and triiodothyroxine (TT3) by radio-immunoassay and thyrotropin (TSH) by immunoradiomebtric assay were measured in the sera of full-term normal newborn infants of both sexes during the first 7 days of life. The results indicated that the mean (+ SE) value of TT4 (11.60 + 0.24 ug/dl) was slightly above our normal adult range while the mean of TT3 (97.48 + 2.84 ng/dl) was within the normal adult range. The levels of serum TSH in the newborns increased markedly at the first day after birth and decreased gradually after 3-7 days of postnatal life with the mean value of 5.02 + 036 nU/l. Thus the normal ranges based on the 95%confidence limits for the results of TT4 (5.00-21.00 ug/dl), TT3 (41.00-195.00 ng/dl) and TSH (0.30-18.00 mU/l) in the neonates were established in order to discriminate euthyroid infant from thyroid disorders of infancy. Serum TT3 and TSH levels of newborns during the first 3 days were found to be significantly higher than those of newborns during 4-7 days (p < 0.005). Moreover, significant sex difference was also obtained in the present study because serum TT4 and TT3 levels of the female infants were statistically higher than those of the male infants, p<0.01 and p<0.05, respectively. In addition, the concentrations of TT4 TT3 and TSH were determined and compared in these newborns following normal labour, caesarean section, gorceps and vaccuum extraction. There was a sharp increase in the mean values of serum TT4 TT3 and utilizing elective caesarean section, while a marked increase in the TSH mean was observed by the methods of forecps extraction and also caesarean section.