Total and Free Serum Thyroid Hormone Concentrations in Fetal and Adult Pregnant and Nonpregnant Guinea Pigs^*

Abstract

Information on total and free serum thyroid hormone concentrations in the adult and fetal guinea pig (Cavia porcellus) is limited. These variables were studied in adult male and female guinea pigs and in pregnant guinea pigs and their fetuses at various times during gestation. Total serum T4 levels in adult males, nonpregnant females, and pregnant females did not differ significantly [range, 2.5 +/- 0.3 to 3.2 +/- 0.8 micrograms/dl (mean +/- SD)]. Similarly, there were no significant differences in the percent free T4 (0.046-0.068%), free T4 (1.26-2.03 ng/dl), total T3 (39-44 ng/dl), the percent free T3 (0.521-0.638%), and free T3 (0.221-0.260 ng/dl) among adult males, nonpregnant females, and pregnant females. rT3 was undetectable in adult male, nonpregnant female, and pregnant female guinea pig serum. T4 values were similar and those for T3 were lower in fetal compared to maternal serum at 45 days of gestation, whereas serum rT3 was detectable in fetal serum. Between 45 and 62 days of gestation, fetal serum T4 increased from 2.5 to 0.3 to 4.3 +/- 1.3 micrograms/dl (mean +/- SD, P less than 0.01), fetal serum T3 remained unchanged, and fetal serum rT3 increased from 5.2 +/- 3.3 to 25.0 +/- 11.4 ng/dl (P less than 0.01). Near term, fetal serum total and free T4 and total rT3 concentrations were significantly higher and total and free T3 concentrations were significantly lower than the corresponding values in maternal serum. Total serum T4 is higher in the guinea pig than in the rabbit, is similar to values in the rat, and is lower than values in man. The free T4 concentration in guinea pig serum is similar to those in humans and rats. The ontogenesis of thyroid hormones differs strikingly in the guinea pig fetus compared to that in the rat fetus and shares many similarities with sheep and human fetal thyroid development.

Repeated Antenatal Glucocorticoid Treatment Decreases Hypothalamic Corticotropin Releasing Hormone mRNA but not Corticosteroid Receptor mRNA Expression in the Fetal Guinea‐Pig Brain

“…The link between glucocorticoid treatment and increased thyroid activity is not clear, although it is quite possible that exposure to synthetic glucocorticoids acts to mature the fetal thyroid gland (48). Thyroid ontogenesis is Fetal glucocorticoid HPA programming 429 advanced in the late gestation fetal guinea-pig and is similar in many respects to that which occurs in humans and sheep in utero (49). In contrast, in the rat, extensive maturation of the thyroid system occurs during early postnatal life (50).…”

American Thyroid Association Guide to Investigating Thyroid Hormone Economy and Action in Rodent and Cell Models

“…Alternatively, sheep have been extensively used as an animal model. The guinea pig is of potential interest as well because its in utero thyroid maturation is much closer to that of the human (477).…”

Effect of Agouti-Related Protein in Regulation of the Hypothalamic-Pituitary-Thyroid Axis in the Melanocortin 4 Receptor Knockout Mouse

“…The details of the assay have been reported previously (17). The Cobra 500 program was used for data reduction and calculation of the RIA results.…”