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This study was designed to investigate the changes in growth and function which occur in the rat thyroid during prolonged TSH stimulation. Animals maintained on the goitrogen aminotriazole were sacrificed together with controls at frequent intervals over a period of 5 months. The levels of serum T3 and T4 and TSH were measured by radioimmunoassay. Functional activity was assessed by measurement of the thyroid/serum iodide ratio (T/S) and growth by measurement of thyroid weight, follicular cell number and follicular cell mitotic activity. Serum T3 and T4 rapidly fell to undetectable levels within 2 weeks. The level of serum TSH rose to a stable 5-fold maximum after 4 weeks. The T/S ratio followed a closely similar pattern rising to a sustained 7-fold maximum. Thyroid weight and follicular cell number increased rapidly for the first few weeks but the growth rate declined progressively, falling almost to zero after 80 days. Mitotic activity rose dramatically to a 30-fold peak after 7 days but then declined almost to normal after 80 days, consistent with the observed change in cell number. The results thus demonstrate a clear dissociation between the functional and proliferative activity of the thyroid follicular cells during prolonged stimulation by a sustained elevation of serum TSH and point to the existence of specific growth regulating mechanisms which limit the mitotic response.

In the course of the last 20 years the world medical literature has reported several instances of the difficulties of interpreting microscopic alterations in thyroid tissue when the patient had been previously treated with radioactive iodine. Nonetheless, these microscopic findings have never been adequately described or produced experimentally. It is generally felt that in the human patient they are not preneoplastic; however, in the rat, many authors have reported the production of thyroid neoplasms by the use of iodine 131.In the present paper the authors have given a clear description of the microscopic changes involved, and report on their experimental production in the rat thyroid.They discuss the significance of these alterations in human patients and in the rat.

Plant cytokinins are a group of growth promoting hormones with pronounced metabolic and regulatory effects not only on plants but also on microbial and animal cells (l). The effects of the N6-substituted derivatives of adenine, especially Zeatin (trans-6-(4-hydroxy-3-methylbut-2-enyl)- aminopurine) and Kinetin (6-furfurylaminopurine) on ultrastructure and on the in vitro response to Thyrotropin (TSH) or 8Br-cyclic AMP were studied in dog thyroid follicular cells.Thyroid slices were incubated in the presence of Zeatin or Kinetin with and without TSH or 8Br-cyclic AMP, after 30min. prior incubation in Earle's solution. Effects on ultrastructure were observed after 30min. to 6hrs. of incubation. The thyroid slices after removal from incubation flasks were fixed immediately in 3%glutaraldehyde-formalin in 0.05M phosphate buffer. Specimens were then treated as described elsewhere (2). Observations were performed with a Siemens IA electron microscope.

In the foetus and neonate the thyroid follicular cells are immature and hence incapable of protecting the organism against an iodide-induced inhibition of hormone synthesis (Wolff-Chaikoff effect). Thus, an iodide excess during pregnancy can lead to transient hypothyroidism in the foetus and neonate. Transient hypothyroidism due to iodide toxicity following amniofoetography in the newborn period is described.

The acute ultrastructural effects of Thyroid Stimulating Hormone (TSH) in vivo or in vitro are well documented. However, little is known about chronic effects of TSH administration. The purpose of the present study was to define the ultrastructural changes induced by chronic TSH administration.Ten Mongrel dogs weighing between 18-20 kg were used in these studies. They were divided in two groups:Control group: (4 dogs) These dogs were administered saline solution, (0.09% NaCl) intramuscular for 10 days.TSH treated group: (6 dogs) These dogs received a daily intramuscular injection of TSH, 15 IU for 10 days.All dogs received 300 mg of Thyroid extract per day for 3 days prior to the operation. Then, they were thyroidectomized on the 11th day. Immediately after removal of the thyroid gland, tissues were fixed in glutaraldehyde 37, in phosphate buffer at pH 7.4 for 2 hrs.

The widespread contamination of the environment with PCB, a compound used extensively by industry in hydraulic and heat transfer fluids as well as plasticizers and solvents in adhesives and sealants, has resulted in detectable tissue levels in a large portion of the human population, domestic animals, and wildlife. Intoxication with PCB produces severe hepatic necrosis, degeneration of lymphoid tissues and kidney, skin lesions, decreased reproductive performance, reduced feed efficiency, and decreased weight gain. PCB also has been reported to reduce the binding of thyroid hormone to serum proteins and enhance the peripheral metabolism of thyroxine with increased excretion of thyroxine-glucuronide in the bile (Bastomsky, Endocrinology 95: 1150-1155, 1974).The objectives of this investigation were (1) to investigate the histopathologic, histochemical, and ultrastructural changes in thyroid FC produced by the acute (4 week) and chronic (12 week) administration of low (50 ppm) and high (500 ppm) doses of PCB to rats, (2) to correlate these alterations to changes in serum immunoreactive thyroxine concentration, and (3) to investigate the persistence of the effects of PCB on the thyroid gland.

Microscopic concretions of barium and sulfur, mixed with natural radium were observed within the follicles of bovine thyroids from widely separated parts of the world. At least 80 per cent of bovine thyroids received from Nigeria, West Africa, and 60 per cent of those studied from Colorado, contained the radioactive concretions; their incidence has been less in other areas. The concretions probably consist of barium sulfate mixed with radium but the mechanisms for incorporation of these elements into thyroid follicles are unexplained. These radioactive bodies cause adjacent thyroid follicular cells to be exposed to relatively intense alpha radiation, but as yet, no biological effects have been ascribed to this radiation.