Graded doses of propylthiouracil, carbimazole, sulphadiazine, potassium perchlorate and potassium thiocyanate were fed to groups of rats under standardized conditions for 2 months. The rats were given 131I 24 hr. before they were killed, and the thyroid weight, the proportions of labelled iodoaminoacids, and the thyroidal iodine content were determined. All the goitrogens produced increase in thyroid size, reduction of thyroidal iodine content, and an elevation of the monoiodotyrosine: di-iodotyrosine (MIT:DIT) ratio. The tri-iodothyronine: thyroxine (T3: T4) ratio was usually increased. Irrespective of the antithyroid compound used, there was a close correlation between the decrease in thyroidal iodine content and the rise in the MIT:DIT ratio.
In further experiments, either thyroid stimulating hormone (TSH) or thyroxine were injected s.c. twice daily for 5 days. TSH decreased the MIT:DIT ratio and increased the amount of labelled T3. Thyroxine increased the MIT:DIT ratio and decreased the amount of T4.
It is suggested that when goitrogens were administered for a prolonged period, the pattern of 131I-labelling of iodoaminoacids was dependent on a balance between inhibition of synthesis caused by the goitrogen and stimulation due to TSH. There was increased production of the physiologically more economical T3 in an attempt to compensate for decreased thyroxine formation.
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