Regulação da síntese e secreção de tireotrofina / Regulation of thyrotropin synthesis and secretion

A secreção de tireotrofina (TSH) é determinada pelo efeito estimulatório do hormônio hipotalâmico estimulador de tireotrofina (TRH) e pela retroalimentação negativa exercida pelos hormônios tireóideos (HT). Superpostos, atuam outros reguladores e aferências do sistema nervoso central. Somatostatina e dopamina inibem a secreção de TSH, já as vias alfa-adrenérgicas centrais são predominantemente estimuladoras e participariam no estímulo da secreção de TSH pelo frio. O estado nutricional modula o eixo através da leptina, por vias diretas e indiretas. O estresse induz redução da secreção de TSH, e discute-se a participação dos glicocorticóides, citocinas e opiáceos. Recentemente, evidenciou-se que fatores locais produzidos na adenohipófise podem atuar de forma autócrina/parácrina, modulando a secreção de TSH. Dentre estes, destacam-se a neuromedina B e o peptídeo liberador de gastrina, que atuam como inibidores locais da secreção de TSH. Discute-se ainda, as alterações do TSH decorrentes da programação neonatal, por hormônios ou desnutrição.

Thyroid function in post-weaning rats whose dams were fed a low-protein diet during suckling

This study was designed to evaluate the thyroid and pituitary hormone levels in post-weaning rats whose dams were fed a low-protein diet during suckling (21 days). The dams and pups were divided into 2 groups:a control group fed a diet containing 22 per cent protein that supplies the necessary amount of protein for the rat and is the usual content of protein in most commercial rat chow, and a diet group fed a lowprotein (8 per cent) diet in which the protein was substituted by an isocaloric amount of starch. After weaning all dams and pups received the 22 per cent protein diet. Two hours before sacrifice of pups aged 21, 30 and 60 days, a tracer dose (0.6 mui) ofl25I was injected (ip) into each animal. Blood and thyroid glands of pups were collected for the determination of serum T4, T3 and TSH and radioiodine uptake. Low protein diet caused a slight decrease in radioiodine uptake at 21 days, and a significant decrease in T3 levels (l28 ñ 14 vs 74 ñ 9 ng/dl, P<0.05), while T4 levels did not change and TSH was increased slightly. At 30 days, T3 and TSH did not change while there was a significant increase in both T4 levels (4.8 ñ 0.3 vs 6.1 ñ 0.2 mug/dl, P<0.05) and in radioiodine uptake levels (0.34 ñ 0.02 vs 0.50 ñ 0.030 per cent/mg thyroid, P<0.05). At 60 days serum T3, T4 and TSH levels were normal, but radioiodine uptake was still significantly increased (0.33 ñ 0.02 vs 0.41 ñ 0.03 per cent/mg thyroid, P<0.05). Thus, it seems that protein malnutrition of the dams during suckling causes hypothyroidism in the pups at 21 days that has a compensatory mechanism increasing thyroid function after refeeding with a 22 per cent protein diet. The radioiodine uptake still remained altered at 60 days, when all the hormonal serum levels returned to the normal values, suggesting a permanent change in the thyroid function.

La glándula tiroides...se autoregula? / The thyroid gland...itself autorregulation?

El aporte de yodo es fundamental para el normal funcionamiento de la glándula tiroides. Analizar el proceso de síntesis de las hormonas tiroideas es de vital importancia para comprender el mecanismo de autoregulación. Esta glándula posee un mecanismo intrínseco de regulación de su propio crecimiento y funcionamiento, el mismo está asociado a la concentración intratiroidea de yoduros y compuestos yodados. Estos compuestos son sintetizados por la misma glándula y parte de ellos son derivados del ácido araquidónico. El análisis de estos hechos permiten entender la etiología de disfunciones tiroideas durante el stress o la ingesta de altas concentraciones de yodo con tejido glandular normal.