Correlated Functional and Morphometric Study of Thyroid Hyperplasia Induced by Iodine Deficiency*

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Abstract
The effect of feeding a low iodine diet (LID) on the thyroids of mice of both sexes for 1–8 weeks was investigated. Morphometric, autoradiographic, and chemical methods were used. Morphologically, the thyroid response to LID could be divided into two successive phases. During the first 4 weeks, the glandular weight remained constant, but changes in the relative volumes of various tissue components occurred, i.e. an increase of the blood vessels and the follicular epithelium, contrasting with a decrease of the colloid. The epithelial height almost doubled. [3H]Thymidine incorporation indexes (TII) increased by the first week in endothelial cells, earlier than in the follicular cells. At all times studied, more than 40% of the TII was related to nonfollicular cells. During the second period (5–8 weeks), the glandular weight increased, but the relative volumes of all of the tissue components, the epithelial height, and the TII were almost stabilized; all of the glandular components grew at a similar rate. Functionally, the evolution was more continuous, and no clear division into phases could be observed. In animals receiving Nal25I 48 h before sacrifice, the [125I]diiodotyrosine content of thyroglobulin (TG) and the [125I]T4 to [125I]T3 ratio decreased. [125I]monoiodotyrosine increased later on. Plasma T4 levels slowly decreased, while T3 levels decreased slightly and transiently before increasing slightly. Although the 127I content of the gland decreased to the same extent in both sexes, the thyroids reacted differently to LID in males and females both morphologically and functionally. In males, the thyroid weight, epithelial height, and TII were larger, the T4 plasma level was lower, and 125I-labeled iodothyronines in TG decreased more rapidly, while [125I]monoiodothyronine increased earlier. These results indicate that 1) different rates of thyroid growth occur during goitrogenesis, 2) changes in blood vessels are observed very early during goitrogenesis and represent a significant part of the glandular hypertrophy, and 3) there are morphological and functional differences between males and females in the response of the thyroid to LID.

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