Pituitary Thyrotroph Function in Hypothyroid Rats

Abstract

The suggestion has been made that the substantial increases in pituitary thyrotrophin (TSH) concentration reported in hypothyroid rats by some workers might be attributable to incomplete surgical thyroidectomy or partial goitrogen block. An attempt, therefore, was made to correlate the severity of hypothyroidism with the storage and release rate of TSH. Female and male Long-Evans rats were made hypothyroid either by surgical thyroidectomy or by administration of propylthiouracil (PTU). Rats were also surgically or chemically thyroidectomized and injected daily with subnormal doses of thyroxine (T4) so as to ameliorate the hypothyroidism. Pituitary and serum levels of TSH were quantified, both by bioassay (stasis tadpole), and radioimmunoassay. The results showed TSH reduced in the pituitary of the severely hypothyroid rat. In both females and males, amelioration of the hypothyroidism with small chronic doses of T4 did not increase bioassayable or immunoassayable TSH stores. Paradoxially, bioassay showed serum TSH titers increased in females inversely proportional to the apparent severity of the hypothyroidism. However, radioimmunoassay of the sera of males, whose hypothyroidism was ameliorated by T4, revealed no significant increases in TSH over thyroidectomy controls. Therefore, the results were unclear as to whether subnormal quantities of T4 increase TSH release from the pituitary. The differences in serum TSH concentration between hypothyroid-ameliorated females and males are not attributable to sex or to differences in the reliability between the bioassay and immunoassay. However, bioassayable increases in TSH stores may perhaps reflect an influence exerted by pituitary gonadotrophins or sex steroids.

A Re-Assessment of the Effect of Thyrotrophin (Tsh) on the Tibial Plate Bioassay for Growth Hormone (Gh)

“…1). The same aliquot of pituitary from analogous normal donors contained approximately 45 to 84 mU bioassayable TSH (Ching 1971). It is not known for certain why the co-admini¬ stration of bovine GH and thyroidectomized rat pituitary extract stimulated greater growth of the epiphysial plate than treatment with normal pituitary extract (Fig.…”

“…The potentiating effect of TSH on GH action was most evident when relatively large quantities of either one or both of these hormones were present. This belief is based in part on the observation that simultaneous ad¬ ministration of GH and anterior lobe extract from thyroidectomized rats which, in other experiments, were found to contain appreciable amounts of TSH (bio¬ assay of 42 to 88 mU per pituitary) (Ching 1971;Cking et al 1974) but no immunoassayable GH (Ching et al 1974) or bioassayable GH (Figs. 1 and 2) increased growth of the tibial plate 2-times faster than equivalent doses of GH alone (Fig.…”

“…It is possible that aside from the effects by other anterior lobe hormones, the higher TSH concentration in normal pituitary extract maintains sufficiently high levels of T4 in blood (estimated at 8.3 fig % by Ching et al 1975) that could initiate endochondral osteogenesis (Riekstniece 8c Asling 1966) and in effect retard the rate of cartilage plate growth. The reduced concen¬ tration of TSH in pituitaries of thyroidectomized rats (Ching et al 1974) would maintain a much lower level of circulating T4 in thyroidectomized or hypo¬ physectomized recipients (Ching et al 1975) and could allow for the greater chondrogenesis in these animals. Thus, low concentrations of T4 may stimulate whereas high levels of circulating T4 retard cartilage growth.…”

Intraocular Development and Adrenergic Innervation of Cortical and Subcortical Brain Areas: Influence of Thyroid Hormone Deficiency

“…The extirpation included the parathyroid glands. Retardation of the body weight increase was used as an indication of successful thyroi dectomy [ 10], The retardation was obvious l week after thyroidectomy, i.e. before grafting, and after 1 month the thyroidectomized animals were only half as heavy as their controls.…”

Acute Decrease of Blood Thyroid Hormone by Isovolemic Exchange Transfusion as a Stimulus for Pulse TSH Release and Its Modifications by CNS Lesions

“…Such a phenomenon was observed several days or weeks after thyroidectomy [Bakke et al, 1968;Ching et al, 1974;Panda and T urner, 1967] or long-term administration of antithyroid drugs [Odell et al, 1967; Ching et al, 1972;G riessen and 1 This study was made as a part of a contract of collaboration between the Czecho slovak Academy of Sciences and the Hungarian Academy of Sciences.…”