Pituitary thyrotrope

Schematic representation of the major steps in the regulation of thyroid hormone secretions and metabolism at five levels, namely, brain, hypothalamus, pituitary thyrotropes, thyroid, and peripheral tissues.

R2. Redding, T. W., and Schally, A. V., Depletion of pituitary thyrotropic hormone by thyrotropin-releasing factor. Endocrinology 81, 918-921 (1967). 

Vagenakis, A. G., Braveiman, L. E., Ferei-doun, A., Partnay, G. I. and Ingbar, S. H. (1975) Recovery of pituitary thyrotropic function after withdrawal of prolonged thyroid suppression therapy. New Engl. J. Med., 293, 681. 

Located in close proximity to the primary capillary plexus in the hypothalamus are specialized neurosecretory cells. In fact, the axons of these cells terminate on the capillaries. The neurosecretory cells synthesize two types of hormones releasing hormones and inhibiting hormones (see Table 10.2). Each of these hormones helps to regulate the release of a particular hormone from the adenohypophysis. For example, thyrotropin-releasing hormone produced by the neurosecretory cells of the hypothalamus stimulates secretion of thyrotropin from the thyrotrope cells of the adenohypophysis. The hypo-thalamic-releasing hormone is picked up by the primary capillary plexus travels through the hypothalamic-hypophyseal portal veins to the anterior pituitary leaves the blood by way of the secondary capillary plexus and exerts its effect on the appropriate cells of the adenohypophysis. The hypophyseal hormone, in this case, thyrotropin, is then picked up by the secondary capillary plexus, removed from the pituitary by the venous blood, and delivered to its target tissue. 

There is no specific information on the effects of ecotoxicants on the fish brain-pituitary axis. Assays for fish TSH are not yet routine. The only current practical measure of thyrotrope activity is from their histological appearance but this is difficult to interpret since it may represent adjustments in negative feedback to maintain plasma free T4 levels. 

In the past decade several lines of evidence have shown that human chorionic gonadotropin (hCG) is a thyroid stimulator and may cause hyperthyroidism. The basis for the thyrotropic effect of hCG is the structural similarity of hCG to TSH (similar a subunits and unique /3 subunits). In hyperthyroid patients with very high hCG levels, serum TSH may be inappropriately detectable owing to the weak crossreactivity of hCG in the radioimmunoassay for TSH. In patients with hyperthyroidism caused by trophoblastic tumors, serum hCG levels usually exceed 300 units/mL and always exceed 100 units/mL. The mean peak hCG level in normal pregnancy is 50 units/mL. On a molar basis, hCG has only 1/10,000 the activity of pituitary TSH in mouse bioassays. Nevertheless, this thyrotropic activity may be very substantial in patients with trophoblastic tumors, whose serum hCG concentrations may reach 2000 units/mL. 

K2. Kirkham, K. E., A new bioassay technique for the measurement in vitro of thyrotropic hormone in serum and pituitary extracts. J. Endocrinol. 26, 259-269 (1962). 

K7. Kumahara, Y., Iwatsubo, H., Miyai, K., Masui, H., Fukuchi, M., and Abe, H., Abnormal thyrotropic substance in the pituitaries of patients with Graves disease. J. Clin. Endocrinol. Metab. 27, 333-340 (1967). 

A comprehensive review of the thyrotropic hormone concentration of the pituitary gland was published by Adams (1946). It was pointed out in that survey that the studies in most animals have shown a tendency for... 

In subsequent studies by Saxton and Loeb (1937) and by Witschi and Riley (1940), it was similarly emphasized that the thyrotropic hormone concentration of the human pituitary gland remains essentially constant throughout life, but in these publications no actual quantitative values were reported. 

Fig. 13. Thyrotropic hormone concentration of pituitary gland of rats at various stages of development. From Turner and Cupps (1939).

Thyrotropic Hormone Concentration of the Anterior Pituitary Gland OF Mice in Animals of Various Ages"... 

Thyrotropin releasing hormone, thyrotropic hormone releasing factor(TRF), thyroliberin Pyr-His-Pro-NH2 (Pyroglutamyi-L-hktidyl-L-prolinamide), M, 262. The identical hormone is found in all hitherto investigated species. Secretion of TRF is promoted by neurotransmitters, e.g. noradrenalin, and inhibited by serotonin. TRF stimulates the anterior pituitary to synthesize and secrete thyrotropin, which in turn stimulates the thyroid gland to secrete thyroxin and triiodothyronin. The latter two hormones exert a negative feedback on the secretion of TRF and thyrotropin. TRF also stimulates the secretion of prolactin and acts as a neurotransmitter in the central nervous system. 

In accordance with known results and the mechanism of action proposed hy us, it seems that the action of relaxin is mainly dependent on thyrotropic hormone of pituitary body. 

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