Thyroid Tumors in the Rat

Spontaneous thyroid tumors derived from the thyroid follicular cell occur in l%-3% of laboratory rats (adenomas and carcinomas combined in a variety of strains of rats aged 2 or more years). 

FIGURE 4.10 Negative feedback loop regulating the function of the gonads. 

Toxicological Risk Assessments of Chemicals A Practical Guide 

There are several ways by which TSH secretion can be increased. An increased hepatic enzyme activity may cause an increased metabolism of thyroid hormones, leading to lower semm hormone levels, which in mm leads to increased secretion of TRH, and subsequently increased TSH secretion. Regarding human relevance, the pathways for regulation of the hypothalamo-pituitary-thyroid axis of rats and humans are similar and the mechanism is relevant for humans, but the human system is far more resistant to perturbation. 

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