Thyroid Hormone Economy in Pregnant Rats Near Term: A “Physiological” Animal Model of Nonthyroidal Illness?*

Abstract

We have studied the changes in thyroid hormone economy that occur in normal pregnant rats between 17-22 days of gestation. T4 and T3 decreased in all extrathyroidal tissues studied, namely plasma, liver, kidney, lung, heart, and skeletal muscle. The exception is the concentration of T3 in cerebral cortex, which remains unchanged, possibly as a consequence of an increase in type II 5'-iodothyronine deiodinase activity. The marked decrease observed in most T4 and T3 pools was not accompanied by a commensurate increase in circulating TSH levels, which at 21 days gestation were either unchanged or actually decreased. The TSH response to TRH appeared to be prolonged. alpha-Glycerophosphate dehydrogenase activity was decreased in the liver, in accordance with its thyroid hormone deficiency. Hepatic type I 5'-iodothyronine deiodinase activity, however, did not decrease, but was slightly increased. Thus, thyroid hormone economy in the pregnant rat near term shows striking similarities with several (but not all) of the changes described in patients with nonthyroidal illness and in several animal models used to study this condition. It is suggested that attenuation of the negative feedback response to the decrease in thyroid hormone pools, leading to low levels of thyroid hormones in most tissues, is the normal physiological response to situations where preservation of energy (and protein) represents a distinct adaptive advantage, as in the case of the pregnant rat and her conceptus.

Septic shock non-thyroidal illness syndrome causes hypothyroidism and conditions for reduced sensitivity to thyroid hormone

“…These findings are similar to those in pregnant rats near term, where decreased levels of T 4 and T 3 were observed in all the studied tissues with the exception of the cerebral cortex, which maintained a normal concentration of T 3 (Calvo et al 1990). Normal levels of T 3 in the frontal lobe of LPS-treated pigs are difficult to explain because T 4 levels were lower than controls, and no differences in DIO3 activity or MCT8 mRNA expression were observed.…”

Effects of the Electromagnetic field, 60 Hz, 3 µT, on the hormonal and metabolic regulation of undernourished pregnant rats

“…However, in this experiment, these dosages were performed at the end of the rat pregnancy. In this period, the dosages are normally lower (Calvo et al, 1990). The data reveals that T 4 and T 3 decreased in all extra thyroidal tissues studied, namely plasma, liver, kidney, lung, heart, and skeletal muscle in normal pregnant rats between 17-22 days of gestation.…”

Effects of marginal iodine deficiency on thyroid hormone production, distribution and transport in nonpregnant and near-term pregnant rats

“…In the iodine-deficient rat, plasma T 4 is decreased and plasma thyrotropin (TSH) increased, while circulating T 3 remains normal (4). In the rat normal pregnancy results in a decrease in both total and free T 4 and T 3 in the plasma (5,6). This leads to reduced concentrations of T 4 and T 3 in the maternal tissues, except for T 3 in the brain (5).…”

“…In the rat normal pregnancy results in a decrease in both total and free T 4 and T 3 in the plasma (5,6). This leads to reduced concentrations of T 4 and T 3 in the maternal tissues, except for T 3 in the brain (5). The clearance of T 4 from plasma is increased markedly, which can be a result of the transport of T 4 to the fetal compartment (6).…”