EFFECTS OF THYROTROPIN ON THYROIDAL WATER AND ELECTROLYTES IN THE CHICK1

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Abstract
Some aspects of electrolyte and water metabolism of thyroid tissue have been studied in the 11/2-day-old chick, with and without thyrotropin. In the untreated chick, each Kg. of thyroid tissue contained 723 gm. of water, 111 meq. of sodium and 60 meq. of potassium. The volume of distribution of radiosodium in the thyroid expanded progressively over a period of 24 hours, ultimately reaching approximately 60 percent of organ weight. By contrast, in nine extrathyroidal tissues studied, equilibration with radiosodium was very rapid. Thirty minutes after the intracardiac administration of TSH, the thyroid gland showed a slight increase in water concentration, a marked increase in the 10-minute uptake of radiosodium, no significant change in the concentration of stable sodium or potassium, and no change in the 10-minute volume of distribution of radio-iodinated human serum albumin. Four hours after TSH, there was a more striking increase in water concentration, no effect on radiosodium uptake, a decrease in the concentration of stable sodium, and an increase in the concentration of stable potassium. The latency of the effect of TSH upon thyroidal radiosodium uptake was 15 minutes or less. The threshold dose was 0.49 milliunits (U.S.P.) per chick. The data indicate the presence in the thyroid of an “extra” extracellular space, the follicular lumen, which is rich in sodium and in which exchange with labelled sodium is slowly accomplished. The action of TSH in the first half-hour is interpreted as an acceleration of the transfer of sodium across one or more thyroidal barriers. The effects of TSH observed after four hours are indicative of cellular hypertrophy and loss of intraluminal colloid.