Effects of extended lactation and 9 alpha-fluoroprednisolone acetate on serum thyroxine and lactational performance.

Abstract

SUMMARY The effects of extending lactation up to 48 days on serum thyroxine (T4), and its relationship to lactational performance, were studied with rats fed a ration containing 1·0 μg I−/g of feed. The effects of chronic administration of 9α-fluoroprednisolone acetate on serum T4 during prolonged lactation were also determined. Lactation was prolonged by substitution of foster litters at 8-day intervals. Serum T4 was measured by a competitive protein-binding assay. Chronic administration of 9α-fluoroprednisolone acetate up to 32 days resulted in a significant increase in serum thyroxine in non-lactating rats. In saline-treated rats lactating up to 48 days, serum T4 was initially reduced by 30% at 16 days, but progressively increased as lactational performance (expressed as gain in weight of pups) decreased. This inverse correlation was attributed to a progressive decline in mammary gland competition for iodine and to a progressive depletion of corticosterone which may have exerted an antithyroid effect initially. Rats treated with 9α-fluoroprednisolone acetate and lactating up to 48 days, had serum T4 levels which were lower than values for corresponding saline-treated lactating rats, but were similar to those levels observed in saline-treated non-lactating rats. This may indicate a higher rate of thyroid hormone utilization in lactating rats treated with 9α-fluoroprednisolone acetate and may also explain the ability of these rats to maintain a better lactational performance than saline-treated lactating rats. Comparisons of each 8-day extended lactation period showed that in all cases lactational performance was greater for mothers treated with 9α-fluoroprednisolone acetate; up to 113% more from 16 to 24 days lactation, up to 153% more from 24 to 32 days lactation, 146% more from 32 to 40 days lactation, and 317% more from 40 to 48 days lactation, despite the fact that by day 48 maternal body weight of lactating rats treated with 9α-fluoroprednisolone acetate was not different from that of saline-treated lactating rats. A hypothesis proposed for the hormonal maintenance of lactation suggests that the maintenance of euthyroid levels of serum T4 due to treatment with 9α-fluoroprednisolone acetate is necessary for the continued prolactin-induced mammary synthesis of fatty acids and nucleic acids. Both glucocorticoids and thyroxine may be rate limiting to milk synthesis.