Answer
Lack of Iodine in the diet results in decreased production of T3 and T4 thyroid hormones. The response of the hypothalamus to the lowered serum levels of thyroid hormones is to increase the secretion of throxine releasing hormone(TRH). TRH is transported to the anterior pituitary(AP) by the hypophyseal portal veins . The effect of this releasing hormone on the thyrotrops of the anterior pituitary is to increase the output of thyroid stimulating hormone (TSH). TSH has several well-known actions : stimulation of growth of follicular cells, promotion of the trapping of Iodine, and increasing the production of T3 and T4. The lack of Iodine means that not enough thyroxine can be produced to slow down the secretion of TRH. So it keeps flowing to the AP which keeps producing elevated levels of TSH. Elevated levels of TSH keep follicular cell mitosis in high gear, so the thyroid gland gets bigger and bigger, resulting in the condition known clinically as goiter.
Work Step by Step
Goiter is an overgrowth of the thyroid gland due to stimulation of growth of follicular cells by elevated levels of serum TSH. In some cases this is the end result of unavailability of dietary Iodine. The consequence of the decrease in the levels of serum Iodine is an increased hypothalamic output of TRH. Thyroxine releasing hormone stimulates the anterior pituitary to pour out large amounts of TSH. TSH stimulates the growth of follicular cells, and it should also increase the production of T3 and T4 which should feed back negatively to slow down or decrease the TSH production. However, since there is little or no Iodine available, production of thyroid hormones cannot be increased, and the thyrotrops keep pouring out unusually high levels of TSH. The result is that the folllicular cells keep multiplying and the thyroid gland grows bigger and bigger. This condition of overgrowth of the thyroid is known as goiter
