[Effect of treatment by thyroxine and propylthiouracil on the rate of formation of norepinephrine in eight peripheral organs of the rat]

Sympathetic nervous system activity in rat thyroid: potential role in goitrogenesis

The role of sympathetic innervation in regulation of thyroid function is incompletely understood. We, therefore, carried out studies in rats utilizing techniques of norepinephrine turnover to assess thyroid sympathetic activity in vivo. Thyroidal sympathetic activity was increased 95% by exposure to cold (4 degrees C), 42% by chronic ingestion of an iodine-deficient diet, and 32% in rats fed a goitrogenic diet (low-iodine diet supplemented with propylthiouracil). In addition, fasting for 2 days reduced sympathetic nervous system activity in thyroid by 38%. Thyroid growth and 125I uptake were also compared in intact and decentralized hemithyroids obtained from animals subjected to unilateral superior cervical ganglion decentralization. Unilateral superior cervical ganglion decentralization led to a reduction in thyroid weight, in 125I uptake by thyroid tissue, and in TSH-induced stimulation of 125I uptake in decentralized hemithyroids. These results suggest that sympathetic activity in thyroid contributes to gland enlargement and may modulate tissue responsiveness to TSH.

Hyperthyroid heart disease

The importance of cardiovascular system involvement in hyperthyroidism has been recognized for many years. In the middle-aged and elderly patient, often with mild but prolonged elevation of plasma thyroid hormones, symptoms and signs of heart failure and complicating atrial fibrillation may dominate the clinical picture and mask the more classical endocrine manifestations of the disease. Pitfalls in diagnosis and the importance of early recognition and treatment are discussed. Despite experimental evidence for a short-term inotropic action of thyroid hormone excess, clinical data support the existence of a reversible cardiomyopathy in hyperthyroidism with impaired contractile reserve. Enhanced myocardial performance at rest primarily reflects the peripheral actions of thyroid hormone excess. Most, if not all, of the cardiac abnormalities return to normal once a euthyroid state has been achieved, although atrial fibrillation may persist in a minority. Optimum treatment requires rapid and definitive antithyroid therapy, usually using a large dose of radio-iodine, and rapid control of heart failure. Systemic anticoagulation is indicated in the presence of atrial fibrillation and should be continued until sinus rhythm has been present for at least three months, either spontaneously or after cardioversion.

Localization of triiodothyronine in nerve ending fractions of rat brain

Radioactive triiodothyronine reaching the rat brain after intravenous administration is rapidly and selectively taken up in the nerve ending fraction. A concentration gradient of radioactivity from brain cytosol to synaptosomes is observed at 5 min, increases linearly over the first hour, and is maintained for at least 10 hr. Radioactivity in the synaptosomes is due to triiodothyronine (90%) plus a single unidentified metabolite (10%). Approximately 85% of the synaptosomal radioactivity is released by osmotic disruption of the particles. The process of selective uptake, concentration, and retention of triiodothyronine in nerve terminals of the rat brain may be related to the sympathomimetic and behavior-altering effects of the thyroid hormones.