T4 degradation rate and plasma levels of TSH and thyroid hormones in ten young bulls during feeding conditions and 48 h of starvation

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Abstract
High-yield lactating cows will often be in a state of energy deficit in early lactation because the milk production exceeds the capacity to consume food. This can result in ketosis. Searching for genetic markers to be used in breeding for more ketosis-resistant animals, it is of interest to study the response to energy deficit in AI[artificial insemination]-bulls. The response can be studied by measuring hormone responses and metabolites. Thyroid hormones are major regulators of metabolic rate. The effect of starvation on thyroid hormones was studied during a starvation period of 48 h in 10 young bulls. Mean thyroxine [T4] degradation rate decreased from K[apparent 1st-order elimination rate constant]/day 0.32 during feeding to 0.23 during fasting. Mean plasma concentration of T4 decreased to 75% of normal, and it was calculated that the mean T4 secretion rate during the starvation period was 24% of normal. Plasma concentration of free T4 decreased to 54% of normal, indicating that the concentration of binding protein was not decreased. T3 [triiodothyronine] and reverse T3 decreased to about 60% of normal. This indicated a parallel decrease in the secretion of all 3 hormones. No evidence was found of an inactivating pathway for T4 as has been shown in humans during starvation. [Thyrotropin] decreased to 85% of normal.

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