Somatostatin reduces 3H-thymidine incorporation and c-myc, but not thyroglobulin ribonucleic acid levels in human thyroid follicular cells in vitro.

The action of somatostatin (SRIH) on 3H-thymidine (thy) incorporation and on c-myc and thyroglobulin RNA levels in a suspension of follicles from normal and goitrous human thyroid was examined. SRIH, at 10(-7) M concentration, inhibited basal thy incorporation (maximally by 4 h lasting for up 24 h), which effect was greater in goiter than in normal thyroid and was also detected in growing adherent epithelial cells. Moreover, in a follicle suspension SRIH prevented TSH-stimulated thy incorporation, both in normal and in goitrous thyroid. Basal expression of c-myc RNA was not affected by SRIH in either tissue, whereas the TSH-stimulated c-myc RNA level was significantly reduced in goiter. No effect of SRIH was observed on basal or TSH-stimulated thyroglobulin RNA levels. SRIH did not alter basal cAMP concentrations in normal or goitrous follicles, but it significantly reduced TSH-stimulated cAMP accumulation both in normal thyroid and in goiter. Overall, our data indicate a direct inhibitory action of SRIH on growth, but not on differentiation, of human thyroid, probably by a mechanism not entirely cAMP dependent.     

Normal-sized thyroglobulin messenger ribonucleic acid in Dutch goats with a thyroglobulin synthesis defect is translated into a 35,000 molecular weight N-terminal fragment

The translation product of the thyroglobulin (Tg) mRNA in Dutch goats with a Tg synthesis defect has been characterized. The Tg mRNA has a normal size of 8.4 kilobases. Translation of goitrous polysomal Tg mRNA resulted, after immunoprecipitation with polyclonal rabbit antigoat Tg antibodies, in a single 35,000 mol wt (Mr) Tg fragment. To characterize the Tg antigens produced in vivo, thyroid hormone release by the goiter was suppressed by injecting T4 sc in newborn goitrous goats. Immunohistochemical studies showed the presence of Tg antigens almost solely in the colloidal lumen. Electrophoresis of the reduced thyroid proteins demonstrated the presence of two Tg fragments with Mr of 40,000 and 32,000, respectively; the latter is probably a breakdown product of the 40,000 Mr fragment. The difference in Mr between the in vivo and in vitro translation products (40,000 and 35,000 Mr, respectively) can be explained by the carbohydrate content (10% wt/wt) of the in vivo product, as was shown by periodic acid-Schiff-positive staining. Using monoclonal antibodies against the hormonogenic sites at the first and last parts of the Tg molecule, we demonstrated that only the first part of the Tg molecule is present. Both in vivo and in vitro 10% of the goitrous Tg mRNA molecule is translated, resulting in an N-terminal Tg fragment that easily aggregates to large S-S complexes in the colloidal lumen of goiter by H2O2 oxidation.     

Freeze fracture morphology of thyroid tight junctions in goats with different thyrotropin stimulation

In the thyroid follicles and blood of goats with congenital hypothyroidism and goiter, abnormal iodoproteins (e.g. iodoalbumin) are found. To study the mechanism involved in the passage of these proteins between the follicles and the blood, the morphology of tight junctions in goiters and normal thyroids of goats was studied by means of freeze fracturing. The T4 and TSH levels of 16 goats were measured by RIA. Based on the TSH levels, the goats could be divided into three groups: a euthyroid group of eight goats with TSH levels between 28 and 55 mU/liter, a hypothyroid group of six goats with TSH levels higher than 199 mU/liter, and an intermediary group of two goats with TSH levels of 120 and 124 mU/liter, respectively. The euthyroid and intermediate animals had normal T4 levels (less than 49 nmol/liter), except for three newborn goats, which had T4 levels above 250 nmol/liter. The hypothyroid goats had T4 levels below 43 nmol/liter. The mean numbers of strands composing the tight junctions were negatively correlated with TSH levels. Hence, the tight junctions of the glands of hypothyroid goats are narrower and are composed of fewer strands than those of normal thyroids. This reduction in tight junction complexity may provide an explanation for the leakage of proteins into the follicles of goitrous glands.     

HEREDITARY CONGENITAL GOITRE WITH THYROGLOBULIN DEFICIENCY CAUSING HYPOTHYROIDISM

The thyroid glands of two hypothyroid goitrous siblings aged 13 and 14 and of a 21-year-old hypothyroid goitrous female were examined. In all three patients a very high thyroid uptake of iodide was observed in the presence of a negative perchlorate discharge test. An abnormally high serum protein bound iodine (12·9–20·0 μg/dl) and low serum T4 concentration suggested the presence of increased serum levels of iodoalbumin. Surprisingly, serum T3 levels were normal or low normal (80–220 ng/dl) in several determinations. Basal serum TSH was elevated and an exaggerated TSH response was observed after TRH. Serum thyroglobulin was undetectable in one patient, low normal in another and in the normal range for the third one. Except for the patient with undetectable Tg the two other subjects slightly increased the serum Tg levels after a bovine TSH injection. Plasma chromatography after a tracer dose of ¹²⁵I disclosed only minute amounts of T3+T4 and MIT+DIT. Studies performed in the homogenized thyroid tissues indicated that these goitrous glands had pronounced decrease of immunoreactive thyroglobulin. The total amount of Tg-like proteins (RIA) in the thyroid soluble protein extract was only 16–122 μg/g (normal: 50–70 mg/g of tissue). Ultracentrifugal studies were unable to demonstrate the presence of mature (18–20S) thyroglobulin. Only one peak (3·6–4·1S) was obtained in the pooled soluble proteins supernatants. Hydrolysis of the homogenates indicated, by subsequent column chromatography, very low relative concentrations of iodotyrosines and iodothyronines and that a relatively large amount of iodide remained associated with subcellular proteins and undigested. The predominant histological pattern was of the intermediary differentiated adenoma type, microfollicular or fetal, with several atypical features and capsular invasion which may suggest malignant change. We conclude that a defective Tg export from the cell to the lumen or an anomaly in the structural gene leading to inadequate translation of Tg mRNA finally results in deficient storage of normal, mature Tg in the colloid with subsequent goitrous hypothyroidism.     

Abnormal cellular localization of thyroglobulin mRNA associated with hereditary congenital goiter and thyroglobulin deficiency.

The goiters in a breed of hypothyroid goats contain only minute amounts of thyroblobulin-related antigens (0.01% of normal value). We have analyzed these goiters for the presence of mRNA coding for thyroglobulin. Using DNA complementary to beef 33S thyroglobulin mRNA as a probe, we found that the mRNA sequence is present in the goat goiter but at a concentration 1/10-1/40 that of normal goat thyroid. Hybrids of cDNA with either goiter or normal thyroid RNA exhibited identical sharp melting curves which suggests that the same RNA sequence is responsible for hybridization in both tissues. Normal goat thyroid contains a population of large membrane-bound polysomes engaged in throglobulin synthesis. In contrast, such polysomes are absent in the goiter. In regard to subcellular distribution, the relative amount of the thyroglobulin mRNA sequences from the goiter in nuclear RNA was 42% of normal, in cytoplasmic RNA was 7% of normal, and in the membrane fraction was only 1-2% of normal. Our results suggest that the lack of thyroglobulin in these goiters is due to a defect in thyroglobulin mRNA which leads to aberrant processing and/or transport of it from its site of synthesis to the endoplasmic reticulum.     

Effect of iodized oil on iodine content, thyroglobulin maturation and on biochemical constituents of endemic goitre in brazil.

The biochemical and morphological changes in thyroid glands were studied in 5 patients with endemic goitre in the region before and one year after the administration of iodized oil. Each patient received a tracer dose of 125-I 3-5 weeks before the biopsy and another tracer of 131-I 12 h before surgery. It was observed that: 1. Iodine deficient goitrous glands had more protein per gram of tissue and that this was corrected by iodination. 2. The goitrous glands exhibited a higher relative proportion of particulate iodoprotein that incorporates more labelled iodine in relation to time of labelling. This abnormal situation is reverted to normal by iodination. 3. A significantly higher proportion of T3 + T4 is synthetized by the glands treated with iodized oil. 4. The increased amount of 127-I per gram of tissue is followed by an increased iodination level of thyroglobulin. 5. The ultracentrifugal pattern of proteins observed for iodine deficient glands differed from the iodinated glands, indicating a failure of maturation of thyroglobulin and possibly a greater than normal degree of heterogeneity. 6. The shift in sedimentation pattern was clearly induced by the increased degree of iodination, as shown by the iodized oil injection. 7. The increased cellularity and the higher rate of protein synthesis in the goitrous glands are reverted to normal after the iodized oil injection.     

Familial goiter in bongo antelope (Tragelaphus eurycerus)

Congenital defects in thyroglobulin (Tg) synthesis in animals have proven to be useful models for the study of Tg synthesis and regulation. Defects in Tg synthesis have been well described in Afrikander cattle, Australia Merino sheep, and goats in The Netherlands. This report describes a study of goiter in a nondomesticated bovine species, bongo antelope (Tragelaphus eurycerus), an African bovid. Three animals housed at the National Zoological Park, Washington, D.C. were studied; two had visible goiters, and a third bongo had microscopic evidence of goiter. Tg extracted from thyroid glands or thyroid colloid from these animals had a high mol wt component that was greater than 220K daltons and differed in apparent mol wt from 19S Tg from domestic cattle. Thyroid extracts also had thyroid albumin; albumin was more than half the total protein in colloid extract. The animals with goiter were euthyroid according to their circulating levels of thyroid hormones.     

Paradoxical effects of thyrotropin on diffusion of thyroglobulin in the colloid of rat thyroid follicles after long term thyroxine treatment

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.     

Intrinsic abnormality of thyroid cells from a patient with a Brobdingnagian goiter

Thyrocytes from normal and goitrous portions of the thyroid gland from a patient with an enormous (635 gm) goiter were cultured in media, and their relative abilities to uptake thymidine, synthesize DNA, and secrete Tg and T4 were assessed. Tg, T4 and the ratio Tg/T4 in media at 72 h were 120 micrograms/dl, 1.6 micrograms/dl and 75 for normal cells, and 410 micrograms/dl and 273 for goitrous cells. DNA synthesis was 30-100% greater in goitrous than normal cells over a period of 72 h indicating abnormally high rates of cell division and growth, but their thymidine uptake was only 20% of that in normal cells. TSH elicited stimulation of thymidine uptake and DNA synthesis both in goitrous and in normal cells, but to a lesser extent in goitrous cells. No evidence of growth stimulating autoantibody activity was detected in the patient's serum. These data indicate that although the ability of goitrous cells to uptake thymidine was impaired, their DNA synthesis was more active than normal thyroid, which may have led to disordered cell growth as evidenced by the enormous goiter size in this patient. Although goitrous cells secreted more than three times as much Tg into media compared to normal, media T4 levels were similar, indicating an increased synthesis yet low iodination of Tg in goitrous cells. There does not appear to be any support for TGSI mediation of the goiter in this patient, but rather, the goiter may have been due to an intrinsic alteration in DNA synthesis resulting in vigorous cell growth.     

Abnormal H2O2 supply in the thyroid of a patient with goiter and iodine organification defect

A 71-yr-old man, clinically euthyroid, with a 570-g goiter causing severe mechanical neck compression underwent thyroidectomy. His total serum T4 level was 1.8 micrograms/dL, T3 was 200 ng/dL, and TSH was 35 microU/mL, and a perchlorate test was markedly abnormal. The excised thyroid tissue had normal peroxidase activity in the tyrosine iodinase and guaiacol assays. [131I]Iodide, given 24 h before surgery, was distributed in thyroglobulin isolated in vitro as follows: monoiodotyrosine, 71.6%; diiodotyrosine, 26.7%; T3, 1.05%; and T4, 0.65%. The [131I]iodide content of the whole thyroid homogenate was 59%. The goiter content of thyroglobulin was 94.7 mg/g tissue. The thyroglobulin reacted normally with antihuman thyroglobulin antiserum. Fresh goiter slices and slices from five normal human thyroid specimens were incubated with 10(-6) M KI and [131I]iodide (tracer) containing medium alone (basal), medium plus 1 mg/mL glucose oxidase (GO), and medium plus 10(-4) M NADPH and 10(-5) M vitamin K3 (NA-K3). The percentages of organic iodine in the slices, measured as protein-bound 131I, were: basal: goiter, 0.8%; normal, 6.9 +/- 1.8% (+/- SE); GO: goiter, 15.1%; normal, 17.4 +/- 3.1%; and NA-K3: goiter, 16.7%; normal, 4.6 +/- 1.14%. We conclude that an abnormal H2O2 supply may be the cause of the iodine organification defect in this goiter.     

Two novel cysteine substitutions (C1263R and C1995S) of thyroglobulin cause a defect in intracellular transport of thyroglobulin in patients with congenital goiter and the variant type of adenomatous goiter

We analyzed the thyroglobulin (Tg) gene of 2 unrelated patients with congenital goiter and the Tg gene of 2 siblings with the variant type of adenomatous goiter. The clinical characteristics of the patients with congenital goiter and the variant type of adenomatous goiter were very similar, except for serum Tg levels, which were less than 15 pmol/L in the patients with congenital goiter, but 117-181 pmol/L in the patients with the variant type of adenomatous goiter (normal, 15-50 pmol/L). The tissue content of Tg in the thyroid glands of all 4 patients was reduced at 0.9-3.8% of total protein (normal, 19-40%). The missense mutation C1263R was detected in the 2 unrelated patients with congenital goiter; the pedigree study showed an autosomal recessive pattern of inheritance. In the 2 siblings with the variant type of adenomatous goiter, the missense mutation C1995S was homozygously detected. In the Tg complementary DNA of 110 normal subjects, the allelic frequencies of the C1263R and C1995S mutations were each less than 0.5%. Also in the normal subjects were detected 35 nucleotide polymorphisms, the insertion of 3 nucleotides, and 1 alternative splicing, each of which was not associated with any specific thyroid disease. From these data, the molecular mechanism of the C1263R and C1995S mutations was elucidated. We first analyzed the carbohydrate residues of C1263R Tg and C1995S Tg. Sensitivity to treatment by endoglycosidase H suggests that C1263R Tg and C1995S Tg were retained in the endoplasmic reticulum (ER). Also, the presence of endoglycosidase H-resistant Tg as well as endoglycosidase H-sensitive Tg in the patients with the variant type of adenomatous goiter suggests that a fraction of C1995S Tg was transported to the Golgi and associated with the mildly increased serum Tg levels. Native PAGE and Western blot analysis with anti-Tg antibody showed that C1263R Tg and C1995S Tg form high mol wt aggregates in the ER. Our results suggest that missense mutations that replace cysteine with either arginine or serine cause an abnormal three-dimensional structure of Tg. Such misfolded Tg polypeptides are retained in the ER as high mol wt aggregates.     

Low levels of thyroglobulin messenger ribonucleic acid in congenital goitrous hypothyroidism with defective thyroglobulin synthesis

We characterized the virtual absence of immunoassayable thyroglobulin (Tg) in the serum and thyroid gland of two siblings (MA, JNA) and one nephew (RSS) from a family without inbreeding or familial goiter. Diagnosis of defective Tg gene expression was based on findings of normal PBI and low serum T4, low or normal serum T3, negative perchlorate discharge test, and virtual absence of the serum Tg response to challenge by bovine TSH. This conclusion was confirmed by analysis of proteins in the goiter extracts. Only minute amounts of immunoassayable Tg were detected by RIA (MA, 0.11; JNA, 0.19 mg/g tissue; compared to 70-90 mg/g in normal thyroid tissue). Gel filtration in Sephacryl S300 showed the absence of a normal Tg peak at 280 nm and concentration of label mostly on albumin. A minor intermediate peak of radioactivity was also detected, with the size of, approximately, normal Tg. Sodium dodecyl sulfate-agarose gel electrophoresis indicated the absence of Tg dimer and monomer, and Western blotting and immunoelectrophoresis confirmed this finding. Dot blot quantification of Tg and thyroid peroxidase mRNA indicated decreased hybridization of the patients' mRNA (MA, 44%; JNA, 63%) with phTgM2 (Tg probe) and increased hybridization (MA, 191%; JNA, 182%) with the pM5 (thyroid peroxidase probe) compared with control thyroid tissue. Dot blot analysis of Tg mRNA from the two siblings weakly hybridized with 3' and 5' Tg probes. RNA analysis by means of Northern transfer showed a clear signal of hybridization with Tg probe (phTgM1) in the 8- to 9-kilobase range, corresponding to the normal size Tg mRNA. No major polymorphisms were noted in Southern blotting, using seven restriction endonucleases. We conclude that no gross alteration of the 5' region of Tg gene was present in these patients. Ultrastructural examination of the thyroid tissue indicated that the rough endoplasmic reticulum was not augmented, nor were the cisternae of rough endoplasmic reticulum dilated. The defect observed in these goiters is diminished tissue concentration of Tg mRNA with defective translation. However, small amounts of functionally active Tg could be synthesized, iodinated, and immediately hydrolized, yielding mostly T3, owing to the intense tissue stimulation by TSH.     

Relationship between thyroglobulin synthesis, iodine metabolism, and histogenesis in the developing rabbit fetal thyroid gland

The biosynthesis of thyroglobulin and its iodination have been studied in the rabbit fetus in relation to the morphogenesis and structure of the thyroid gland. Sixteen- to 28-day-old fetuses were injected intraperitoneally in vivo through the uterine wall with [³H]leucine and ¹²⁵I. Tritium-labeled proteins were analyzed by sucrose gradient centrifugation and dise electrophoresis, in combination with immunological techniques. Thyroglobulin was estimated by radioimmunoassay, and ¹²⁷I by chemical determination. The first definite follicles were seen in electron micrographs on day 18. Colloid is not stored in large amounts until day 22.     

Studies on the thyroidal defect in an atypical form of Pendred's syndrome.

A 52-yr-old woman and her 47-yr-old brother both had large goiters, short stature, nerve deafness, and mild intellectual impairment. Serum levels of thyroid hormones were normal (sister) on borderline low (brother), urinary 127I levels were normal, and perchlorate discharge tests were negative. After adminstration of 125I, we found normal uptakes and retention of istotope by the thyroid, normal serum half-lives for PB 125I, and absence of labelled iodotyrosines in serum or urine. Particulate fractions from both glands had peroxidase activity by in vitro tests for guaiacol oxidation and tyrosine iodination. Samples from different parts of the sister's gland showed a correlation between peroxidase activity in vitro and radioiodine uptake in vivo. We made three separate preparations of thyroglobulin from the sister and one from the brother by gel filtration on 4% agarose. These were similar to each other and to thyrolobulin samples from normal humans when examined by gel electrophoresis both before and after reduction and alkylation. However, the three thyroglobulins from the same gland differed significantly from one another in their composition of most amino acids. This finding indicates that there can be heterogeneity in the protein portion of thyroglobulin, and since the three samples of thyroglobulin were from the same individual, this heterogeneity was probably not of genetic origin. In these patients we could not demonstrate any of the well-established biochemical defects associated with familial goiter. An abnormality in thyroglobulin structure remains in distinct possibility, but we must defer conclusions until there is more known about thyroglobulin heterogeneity and its relationship to hormone biosynthesis.     

Histone deacetylase inhibitors restore radioiodide uptake and retention in poorly differentiated and anaplastic thyroid cancer cells by expression of the sodium/iodide symporter thyroperoxidase and thyroglobulin

Iodide uptake by the thyroid is mediated by the sodium/iodide symporter. Upon iodide uptake, thyroperoxidase catalyzes iodination of tyrosine residues in thyroglobulin, retaining iodide within thyroid follicles. Dedifferentiation-induced loss of these functions in cancers, rendering them unresponsive to radioiodide, occurs with most poorly differentiated and anaplastic tumors. We focused on the histone deacetylase (HDAC) inhibitors (HDACI) as a way to induce differentiation of thyroid cancer cells. We assessed re-expression of thyroid-specific genes mRNA induced by HDACI using quantitative RT-PCR and immunostaining in poorly differentiated papillary and anaplastic thyroid cancer cells. HDACI induced expression of thyroid-specific gene mRNAs and proteins, and accumulation of radioiodide through iodination of generic cellular proteins were detected. HDACI-treated tumors could specifically accumulate (125)I as revealed by imaging experiments and radioiodide concentration in vivo. In an attempt to determine the mechanism by which these gene expressions occurred, we detected the inhibition of protein synthesis by cycloheximide, which up-regulated the expression of thyroperoxidase and thyroglobulin mRNA in HDACI-treated cells and down-regulated that of sodium/iodide symporter mRNA. Together, our results suggest that HDACI-induced expression of thyroid-specific genes, some of which is mediated by some protein synthesis, may contribute to development of novel strategy against thyroid cancer.     

The loss of the chloride channel, ClC-5, delays apical iodide efflux and induces a euthyroid goiter in the mouse thyroid gland

Genetic inactivation of ClC-5, a voltage-gated chloride channel prominently expressed in the kidney, leads to proteinuria because of defective apical endocytosis in proximal tubular cells. Because thyroid hormone secretion depends on apical endocytosis of thyroglobulin (Tg), we investigated whether ClC-5 is expressed in the thyroid and affects its function, using Clcn5-deficient knockout (KO) mice. We found that ClC-5 is highly expressed in wild-type mouse thyroid ( approximately 40% of mRNA kidney level). The protein was immunolocalized at the apical pole of thyrocytes. In Percoll gradients, ClC-5 overlapped with plasma membrane and early endosome markers, but best codistributed with the late endosomal marker, Rab7. ClC-5 KO mice were euthyroid (normal T4 and TSH serum levels) but developed a goiter with parallel iodine and Tg accumulation (i.e. normal Tg iodination level). When comparing ClC-5 KO with wild-type mice, thyroid 125I uptake after 1 h was doubled, incorporation into Tg was decreased by approximately 2-fold, so that trichloroacetic acid-soluble 125I increased approximately 4-fold. Enhanced 125I- efflux upon perchlorate and presence of 125I-Tg as autoradiographic rings at follicle periphery demonstrated delayed iodide organification. Endocytic trafficking of 125I-Tg toward lysosomes was not inhibited. Expression of pendrin, an I-/Cl- exchanger involved in apical iodide efflux, was selectively decreased by 60% in KO mice at mRNA and protein levels. Thus, ClC-5 is well expressed in the thyroid but is not critical for apical endocytosis, contrary to the kidney. Instead, the goiter associated with ClC-5 KO results from impaired rate of apical iodide efflux by down-regulation of pendrin expression.     

Effects of iodide and thyroxine on iodine-deficient mouse thyroid: a morphological and functional study

The effects of iodide and thyroxine (T4) on female mice fed a low iodine diet (LID) for 8 weeks were analysed by morphological, stereological and biochemical methods. Iodide was given at a dose of 10 micrograms/day (HID) or 1 microgram/day (MID), either alone or together with daily injections of 1 microgram T4 for 8 or 40 days. With HID, the thyroid weight and the numbers of follicles and cells remained higher than in controls, although cell necrosis occurred. Colloid volume increased and iodine was stored within the gland: a colloid goitre with non-functioning follicles was produced. With MID, the glands resumed an almost normal appearance. With T4 and LID, progressive normalization occurred, but after 40 days thyroid weight and numbers of follicles and cells remained higher than in controls. Glandular iodine content slowly increased and reached control value. The proportions of 125I-labelled tri-iodothyronine (T3) and T4 in thyroglobulin were reduced. With T4 and HID, the glands resumed a normal appearance. Neither necrosis nor folliculoneogenesis was noted. The proportions of 125I-labelled T3 and T4 in thyroglobulin were reduced, but T3 and T4 serum levels were higher than with HID. With T4 and MID, a normal state was obtained as early as day 8. After 40 days the gland was morphologically and functionally inactive. In conclusion, the association of T4 and iodide seems to be the best way to obtain a rapid and complete involution of thyroid hyperplasia. The administration of T4 prevents the deleterious effects of an excess of iodine on follicular cells, and causes the gland to enter a slow-functioning state.     

Increased follicular heterogeneity in experimental colloid goiter produced by refeeding iodine excess after thyroid hyperplasia

Delayed morphological changes induced in mouse hyperplastic thyroid by refeeding iodine were analyzed by light and electron microscopy, stereology, and autoradiography. Thyroid hyperplasia was induced by a low iodine diet supplemented with 0.25% propylthiouracil for 10 days. Involution was obtained by discontinuing the propylthiouracil and returning either to a moderate iodine diet [(MID) 1 microgram I/day] or to an iodine-rich diet [(HID) 10 micrograms I/day] for 40 days. In other experiments, three cycles of hyperplasia (8 days) and subsequent involution (8 days) with MID or HID were brought about. Control animals were fed MID or HID. All animals were killed when 12-14 weeks old after injection of 10-50 microCi 125I. Double labeling, with repeated injections of [3H]thymidine from day 0 to day 7 of involution followed by 125I injection 4 h before killing, was also performed. When involutions were performed with MID, most morphological variables returned to control values. However, when involution was brought about with HID, the glandular weight, the number of follicles, and the relative volume of follicular lumina remained larger than in controls. Moreover, the 125I-labeling pattern of the follicles was altered. The proportions of unlabeled, and unevenly or partly labeled, follicles, which were fewer than 5% in control groups, represented 25-35% of all follicles after involution with HID, whereas they were unchanged with MID. In unlabeled follicles the epithelium was flattened, with a reduced number of microvilli. Partly labeled follicles were of two types. In some follicles a persistent ring reaction was observed, suggesting an abnormally slow mixing of thyroglobulin. In others, the 125I labeling was restricted to areas adjacent to the apex of a reduced number of cells, suggesting that some cells were iodinating thyroglobulin, whereas others were not. There was no relationship between the follicular 125I labeling and the frequency of [3H]thymidine-labeled cells. These results indicate that refeeding iodine excess after hyperplasia leads to the formation of a colloid goiter with new follicles, and to an increased heterogeneity of iodine metabolism among follicles and among cells.