Autoradiographs of human goiters demonstrate that the speed of diffusion of newly iodinated thyroglobulin (Tgb) molecules through the colloid space may vary widely from one follicle to another. Since the mechanisms which govern the mixing of the colloid are unknown, we investigated the effect of TSH on these processes in rat thyroid glands. Autoradiographs were prepared from thyroids of rats killed 1 h after 125I or 4 h after [3H]leucine injection. In animals treated with T4 for 2 days, 70% of all follicles showed ring labeling of the colloid periphery with both isotopes, indicating slow mixing of newly synthesized and newly iodinated Tgb molecules with preexisting ones. TSH markedly enhanced the mixing process, thereby diminishing the incidence of ring reactions to roughly 10% of all follicles. These results were expected. Unexpected however, was, the nearly total absence of rings in thyroids treated with T4 for 25 days. Semiquantitative autoradiography revealed a higher absolute number of both newly iodinated and newly synthesized Tgb molecules in the core of follicles in chronically suppressed compared to acutely suppressed thyroids. Moreover, after chronic T4 pretreatment, the effect of TSH on diffusion was the opposite of that observed in acutely T4-treated glands, since 0.5 IU TSH injected twice daily between days 21 and 25 caused the reappearance of 125I and [3H]leucine labeled rings in 44% and 33%, respectively, of all follicles. We conclude that acute TSH suppression slows intraluminal diffusion of thyroglobulin molecules and acute TSH injection accelerates the mixing process, whereas, in contrast, chronic TSH suppression improves and acute TSH action on chronically suppressed follicles impairs diffusion. Therefore, the impact of TSH-mediated processes on the hydrodynamic properties of colloid, and thereby on the intraluminal iodination and coupling process, is more complex than hitherto thought.

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in this paper additional information on low molecular weight, soluble, intrathyroidal iodocompounds with slow metabolic rate is provided. These compounds have previously been localized autoradiographically within the follicular cells. Radioiodide was administered to rats on a normal iodine intake (6–7 μg/day) for 80 days to approach isotopic equilibration of the intrathyroidal iodine with the dietary radioiodide. When the isotope was omitted from the diet the intrathyroidal radioiodine was released with an apparent half-life of approximately 12 days. When the individual soluble components carrying radioiodine were analyzed after separation on Sephadex G-200, different apparent half-lives were found, the half-life of thyroglobulin (Tgb) being roughly 10 days and that of the low molecular weight iodocomounds being in the order of 60 to 100 days or more. In addition to the soluble low molecular weight iodocompounds, the radioactivity in the particulate fraction increased by 100 % during the tracer washout when compared to Tgb and the total soluble fraction. The soluble slow turnover iodocompounds contained a higher percentage of carbohydrate and total iodine than Tgb, while the relative amounts of each sugar analyzed (hexoses, fucose, hexosamine and sialic acid) were close to those in Tgb. Sephadex G-25 chromatography of the low molecular weight iodocompounds obtained after Sephadex G-200 separation resulted in the separation of 4 peaks. Two peaks identified as iodopeptides could be further analyzed. The carbohydrate composition of these peptides was similar to that of 2 glycopeptides obtained after in vitro enzymatic hydrolysis of purified Tgb with pronase. Slow equilibration with radioiodine, long apparent intrathyroidal half-life and carbohydrate content similar to that of Tgb, taken together with previously published data on intracellular localization of soluble intrathyroidal iodocompounds, suggest that the low molecular weight iodocompounds are products of in vivo hydrolysis of engulfed Tgb droplets.

The hormone content of in vitro iodinated thyroglobulin is a constant fraction of the iodine content of the protein under most, but not all, experimental conditions. In contrast, in vivo iodinated human thyroglobulin may contain as little as 10% or as much as 50% of its total iodine in the T4 molecules. Surprisingly, in some poorly iodinated thyroglobulins up to 30% of the iodine may be found in T4. The mechanism of the apparent dissociation between iodination and coupling efficiency (i.e. percentage of total iodine present as iodothyronines) may be dilution of pre-existing high iodinated thyroglobulin stores by non-iodinated prethyroglobulin. This hypothesis was tested by feeding rats PTU and KClO4 for 9 days and injecting T4 during the last 2 days. Thyroglobulin iodination dropped from 0.9 to 0.13% but the coupling efficiency remained unchanged at 25.7 and 23.9%. The exchange of highly iodinated thyroglobulin molecules for non-iodinated ones is one of the two in vivo mechanisms suggested so far which can lead to an apparent dissociation of thyroglobulin iodination and couling efficiency.

Pharmacological concentrations of iodide (> 1 x 10(-6) mol/l) are known to inhibit thyroid follicular cell growth in vitro. However, the inhibitory effect varies widely, depending on experimental conditions, and usually does not exceed 50%. We demonstrate that iodide (10(-4) mol/l) inhibits the growth of FRTL-5 cells in different passages by 11-67%. When five subclones of FRTL-5 cells were compared to the wild type, iodide-induced growth inhibition varied between 25% and 46%. The individual degree of inhibition of each clone was reproducible in two subsequent passages, suggesting that it is a stable constitutive trait. When FRTL-5 cells were grown first in three-dimensional clusters and then transplanted onto nude mice with high endogenous thyrotropin secretion, iodide at a serum concentration of less than 5.7 x 10(-7) mol/l nearly completely blocked cell replication in the transplants but not in the mice's own thyroid. Five cell lines, prepared from autonomously growing hyperthyroid feline multinodular goiters, were nearly completely resistant to the growth-inhibitory effect of iodide. These observations suggest that the sensitivity towards the growth-inhibiting effect of iodide is a highly variable, stable trait of each thyrocyte, even in cloned cell populations. Some FRTL-5 cells and, even more so, cells prepared from autonomously growing nodular feline goiters are resistant constitutively to the growth-inhibiting effect of iodide.(ABSTRACT TRUNCATED AT 250 WORDS)