Thyroidal regulation of nuclear tri-iodothyronine receptors in the developing rat testis

Hypothyroidism impairs antioxidant defence system and testicular physiology during development and maturation

In the present study, effects of transient hypothyroidism (from birth to 30 days) and persistent hypothyroidism (from birth to 90 days) on testicular antioxidant defence system of mature rats were compared in order to know the role of hypothyroidism induced oxidative stress in testicular development and maturation. Rats were made hypothyroid by feeding lactating mothers and adult rats with 0.05% 6-n-propyl thiouracil (PTU) in drinking water. PTU treatment for 30 days or for 90 days to rats from birth resulted in a decrease in body weight at the age of 90 days in comparison to the controls. The testicular germ cell counts were significantly decreased in persistent hypothyroid rats whereas they were increased in the transient hypothyroid rats. However, a significant reduction in the number of live sperms in epididymis of both 30 day and 90-day PTU treated rats was noticed on 90 days of age. Mitochondrial lipid peroxidation (LPx) levels were decreased in transient hypothyroidism whereas LPx and protein carbonylation were elevated during persistent hypothyroidism in the testis. Reduced testicular superoxide dismutase (SOD), catalase and glutathione reductase (GR) and glutathione peroxidase (GPx) activities were marked during transient hypothyroidism. In contrast, an elevation in SOD (PMF) and catalase activities with a significant decline in GPx and GR activities was found following persistent hypothyroidism. Marked histological changes were observed in the testis of both experimental groups. These results suggest a direct regulatory role of thyroid hormone on testicular physiology and antioxidant defence system during development and maturation.

Thyroid hormone, glucocorticoids, and prolactin at the nexus of physiology, reproduction, and toxicology

A symposium at the 2003 Annual Meeting of the Society of Toxicology brought together an expert group of endocrinologists to review how non-reproductive hormones can affect the endocrine system. This publication captures the essence of those presentations. Paul Cooke and Denise Holsberger recapitulate the evidence of how thyroid hormones affect male and female reproduction, and reproductive development. Ray Witorsch summarizes the many effects of glucocorticoids on the reproductive system. Finally, Paul Sylvester reviews the mechanism of action of prolactin, and reminds us that this ancient hormone has many functions beyond lactation.

Thyroid hormones: their role in testicular steroidogenesis

Thyroid hormones are important for growth and development of many tissues. Altered thyroid hormone status causes testicular abnormalities. For instance, juvenile hypothyroidism/neonatal transient hypothyroidism induces macroorchidism, increases testicular cell number (Sertoli, Leydig, and germ cells) and daily sperm production. Triiodothyronine (T3) receptors have been identified in sperm, developing germ cells, Sertoli, Leydig, and peritubular cells. T3 stimulates Sertoli cell lactate secretion as well as mRNA expression of inhibin-alpha, androgen receptor, IGF-I, and IGFBP-4. It also inhibits Sertoli cell mRNA expression of Müllerian inhibiting substance (MIS), aromatase, estradiol receptor, and androgen binding protein (ABP) and ABP secretion. T3 directly increases Leydig cell LH receptor numbers and mRNA levels of steroidogenic enzymes and steroidogenic acute regulatory protein. It stimulates basal and LH-induced secretion of progesterone, testosterone, and estradiol by Leydig cells. Steroidogenic factor-1 acts as a mediator for T3-induced Leydig cell steroidogenesis. Although the role of T3 on sperm, germ, and peritubular cells has not yet been completely studied, it is clear that T3 directly regulates Sertoli and Leydig cell functions. Further studies are required to elucidate the direct effect of T3 on sperm, germ, and peritubular cells.

Are germ cell factors essential in the testicular enlargement after neonatal hypothyroidism recovery? A study using W/Wv mutant mice model

We examined the issue of whether germ cell factors are required for testicular enlargement that occurs after recovery from neonatal hypothyroidism. Experiments were performed using W/Wv mutant mice (lacking germ cells) and normal mice (ICR). The pups in experimental group (neonatal hypothyroid) received 6 propyl 2-thio-uracil (PTU) treatment, administered by adding 0.1% (w/v) to the water provided to the mother from day 1 of birth through day 25 postpartum, while the pups of control group received drinking water only. Mice were sacrificed at the age of day 25, 50 and 90, in the case of ICR mice, or at day 25 and 90 in the case of W/Wv mutant mice. In both groups, early hypothyroidism caused a partial recoverable decrease in body growth and testicular development. Both ICR and W/Wv mutant mice, those recovered from neonatal hypothyroidism showed an increase in testis weights, the number of Sertoli cells, and the diameter of the semniferous tubules. This study demonstrates that neonatal hypothyroidism led recovery caused testicular enlargement not only in ICR mice but also in germ cell depleted W/Wv mutant mice. Hence these findings deny direct involvement of the germ cell factors in the process of testicular enlargement in recovered mice even in vivo, and reaffirm the notion that thyroid hormone directly regulates the dynamics of Sertoli cell maturation.

Aromatase expression in prepuberal Sertoli cells: effect of thyroid hormone

Aromatase activity has recently been assumed as a Sertoli cell functional maturation marker since it is maximally expressed in prepuberal age then it dramatically decreases at puberty and is virtually absent in adult age. Neonatal hypothyroidism is associated with a prolonged proliferation of Sertoli cells. This immature stage persists concomitantly with a dramatic enhancement of aromatase activity reversed by triiodothyronine (T3) either in vivo or in vitro administration. Therefore, in the present study, after immunolocalisation of aromatase in the cytoplasm of cultured Sertoli cells as well as in testis section, we investigate the regulatory effects of T3 in the same cells just at the age when aromatase activity is reported to be maximally expressed. In this aim, the effects of thyroid hormone have been evaluated in 2-weeks-old rats, in basal condition and upon stimulation with dibutyryl cyclic AMP [(Bu)(2)cAMP] by simultaneously analysing three functional levels of aromatase, mRNA expression; protein content; enzymatic activity. Western-blot analysis of Sertoli cell extracts revealed a protein, which co-migrated with a 55 kDa protein detected in human placenta used a positive control. The presence of a functional P450 aromatase protein in purified Sertoli cells was confirmed by the measurement [3H]H(2)O released after incubation with [1beta-(3)H]androst-4-3,17-dione. At the dose used, T3 down-regulates basal aromatase activity, while aromatase mRNA expression was apparently not inhibited. It is noteworthy that aromatase content pattern evaluated by Western blot analysis did not tightly parallel the aromatase activity pattern which clearly displays the inhibitory effects of T3, in basal condition ad upon (Bu)(2)cAMP stimulation, simulating FSH stimulation. The detection of mRNA altered transcript coding for putative protein lacking both aromatic and heme-binding regions upon T3 treatment and unable to convert androgens into estrogens, provides a reasonable explanation for the observed discrepancies between aromatase protein pattern, P450arom mRNA levels and aromatase activity. The authors conclude that although the altered transcript induced by prolonged exposure to T3 is a mechanism by which T3 may down regulate aromatase activity, it cannot be ruled out a direct effect of this hormone at the transcription levels since a recognisable emisite for potential TR(s) binding is located in the promoter region of aromatase gene. Thus a further investigation on T3 modulator role on aromatase gene promoter should be pursued even utilising higher doses of T3.

Interaction of thyroid hormone and steroidogenic acute regulatory (StAR) protein in the regulation of murine Leydig cell steroidogenesis

The steroidogenic acute regulatory (StAR) protein, a novel phosphoprotein, is a crucial factor involved in intramitochondrial cholesterol transportation, the rate-limiting step in steroidogenesis. The present investigations were undertaken to elucidate involvement of thyroid hormone and StAR protein in the regulation of steroidogenesis in mouse Leydig cells. Treatment of cells with triiodothyronine (T(3)) coordinately augmented the levels of StAR protein, StAR mRNA, and steroid production, and these responses were progressively dependent on expression of steroidogenic factor 1 (SF-1). With regard to steroidogenesis and StAR expression, the T(3) response requires both on-going mRNA and protein synthesis. In addition, the effects of T(3) were acutely modulated at the steroidogenic machinery and luteinizing hormone receptor (LHR) function, while these levels were suppressed following longer periods of exposure to T(3). Furthermore, the inhibition of SF-1 expression by DAX-1 markedly abolished T(3)-mediated StAR expression in a time frame, which was consistent with decreased steroid biosynthesis. Specific involvement of SF-1 was further confirmed by assessing the 5'-flanking region of the mouse StAR gene, which identified a region between -254 and -110 bp that was essential for T(3) function. Importantly, it was found that the SF-1 binding site at position -135 bp of the 5'-flanking region was greatly involved in T(3)-mediated reporter activity. Electrophoretic mobility shift assays (EMSA) also demonstrated involvement of SF-1 in T(3) function. The relevance of T(3)-mediated LHR function was investigated in mice rendered hypo-and hyperthyroid, which accounted for up-regulation in the former and down-regulation in the latter group, respectively. These findings demonstrate a key role of thyroid hormone in maintaining mouse Leydig cell function, where thyroid hormone and StAR protein coordinately regulate steroid hormone biosynthesis.

Assessment of mechanisms of thyroid hormone action in mouse Leydig cells: regulation of the steroidogenic acute regulatory protein, steroidogenesis, and luteinizing hormone receptor function

Recently, we demonstrated that triiodothyronine (T(3)) stimulated steroid hormone biosynthesis and steroidogenic acute regulatory (StAR) protein expression in mLTC-1 mouse Leydig tumor cells through the mediation of steroidogenic factor 1 (SF-1). We now report a dual response mechanism of T(3) on steroidogenesis and StAR expression, and on LH receptor (LHR) expression and binding in mLTC-1 cells. T(3) acutely (8 h), induced a 260% increase in StAR messenger RNA (mRNA) expression over the basal level which was coincident with an increase in progesterone (P) production. In contrast, chronic stimulation with T(3) (beyond 8 h), resulted in an attenuation of StAR expression and P production. This attenuation was most likely caused by a decrease in cholesterol delivery to the inner mitochondrial membrane as demonstrated by incubations with the hydrophilic steroid precursors, 22R hydroxycholesterol and pregnenolone, which restored P synthesis. In similar studies, chronic treatment with T(3) increased the levels of cytochrome P450scc mRNA by 83%, whereas those of cytochrome P450 17alpha-hydroxylase and 3ss-hydroxysteroid dehydrogenase decreased. The diminished response in steroidogenesis following chronic T(3) exposure was not a result of alterations in StAR mRNA stability, but rather was due to inhibition of transcription of the StAR gene. Similar acute stimulatory and chronic inhibitory responses to T(3) were found when LHR mRNA expression and LHR ligand binding were examined. Transfections with an LHR or StAR promoter/luciferase reporter construct demonstrated that a 173-bp fragment of the LHR promoter containing an SF-1 binding motif was involved in T(3) response, as was the SF-1 recognition site at -135 bp in the StAR promoter. Furthermore, the importance of SF-1 in T(3) function was also verified employing mutation in the bases of SF-1 sequences using electrophoretic mobility shift assays. The potential physiological relevance of these findings was demonstrated when similar responses were obtained in mice rendered hypo and hyperthyroid. Collectively, these observations further characterize the thyroid-gonadal connection and provide insights into the mechanisms for a dual regulatory role of thyroid hormone in Leydig cell functions.

High-affinity binding of T3 to epididymis nuclei

Thyroid hormones play an important role in epididymal function. Hypothyroid animals experience a significant decrease in the number and forward motility of sperm and a remarkable impairment of epididymal morphology. However, it is yet unknown if such activity is due to direct actions of iodothyronines on the target epididymis. The eventual identification of T3 receptors in the nucleous of epididymal cells becomes relevant. For this reason, the authors searched for specific high-affinity binding of T3 to these nuclei. Twenty prepuberal male Wistar rats were used. The testes and epididymis were approached as one unit through a scrotal incision. The fat-free epididymides were subjected to standard techniques to prepare the nuclei for incubations with 125I-T3 concentrations, ranging from 0.5 x 10(-9) to 2.0 x 10(-11) M. Calculations of association constants and binding capacities were performed according to Scatchard. A single binding site with a Ka of 3.06 +/- 0.6 x 10(9) M(-1) or Kd of 3.26 +/- 0.6 x 10(10) M and a maximal binding capacity of 0.11 +/- 0.02 pmol T3/microg DNA were observed. It is concluded that these nuclei contain a specific T3 receptor. This finding strongly suggests that thyroid hormones have direct effects on the epididymis.

Endocrine-disrupting chemicals: prepubertal exposures and effects on sexual maturation and thyroid function in the male rat. A focus on the EDSTAC recommendations. Endocrine Disrupter Screening and Testing Advisory Committee

Puberty in mammalian species is a period of rapid interactive endocrine and morphological changes. Therefore, it is not surprising that exposure to a variety of pharmaceutical and environmental compounds has been shown to dramatically alter pubertal development. This concern was recognized by the Endocrine Disrupter Screening and Testing Advisory Committee (EDSTAC) that acknowledged the need for the development and standardization of a protocol for the assessment of the impact of endocrine-disrupting compounds (EDC) in the pubertal male and recommended inclusion of an assay of this type as an alternative test in the EDSTAC tier one screen (EPA, 98). The pubertal male protocol was designed to detect alterations of pubertal development, thyroid function, and hypothalamic-pituitary-gonadal (HPG) system peripubertal maturation. In this protocol, intact 23-day-old weanling male rats are exposed to the test substance for 30 days during which pubertal indices are measured. After necropsy, reproductive and thyroid tissues are weighed and evaluated histologically and serum taken for hormone analysis. The purpose of this review was to examine the available literature on pubertal development in the male rat and evaluate the efficacy of the proposed protocol for identifying endocrine-disrupting chemicals. The existing data indicate that this assessment of puberty in the male rat is a simple and effective method to detect the EDC activity of pesticides and toxic substances.

Ontogeny and regulation of variant thyroid hormone receptor isoforms in developing rat testis

High affinity-low capacity nuclear triiodothyronine (T3) receptors (TRs), identified as a product of c-erbAalpha proto-oncogene, are expressed in prepubertal rat Sertoli cell. At this age, exogenous T3 treatment as well as hypothyroidism affects Sertoli cell functions. We examined the ontogenetic expression pattern of TRs in the rat testis. Northern analysis confirms that TRs are expressed at high level from fetal development until prepubertal period. RNase protection analysis demonstrates that TRalpha2, the variant isoform of TRalpha1, is constitutively expressed at all ages, while TRalpha3 is absent in the adult gonad. While TRalpha1 and TRalpha2 expression declines during development, Rev-erbAalpha (Rev), the antisense mRNA encoded by the same c-erbAalpha genomic locus, increases beginning 5 days after birth and maximizing in adulthood. TRalpha1, TRalpha2, and Rev mRNAs do not appear to be directly regulated by thyroid hormone in testis; however, short-term neonatal hypothyroidism leads to the expression of TRalpha1 and its variant in adult testis, which is absent in control coeval animals. Thus, during development of rat testis, the levels of messages of genes encoded in the c-erbAalpha. genomic locus have different ontogenetic control. The ontogenetic profile of TRalpha1 and its variant isoforms within the seminiferous epithelium suggests that these receptors are involved in the differentiation of the male gonad.

Impact of neonatal onset hypothyroidism on Sertoli cell number, plasma and testicular interstitial fluid androgen binding protein concentration

The impact of neonatal onset hypothyroidism from day 1 postpartum through different postnatal developmental events on rat testis was studied in vivo. Hypothyroidism was induced in neonates by feeding the lactating mother or directly with 0.05% methimazole (MMI) through drinking water from the day of birth and were killed at day 10, 15, 30, 40 and 60 postpartum. Hypothyroidism was confirmed by radioimmunoassay of thyroid hormones and TSH. Sertoli cell number, plasma and testicular interstitial fluid (TIF) androgen binding protein (ABP) concentration was quantified. Sertoli cell number was consistently decreased in all hypothyroid rats. Plasma ABP was also decreased irrespective of the duration of hypothyroidism. Unlike plasma ABP, TIF ABP concentration in hypothyroid rats increased at day 10, and 15 postpartum and decreased in other age groups. Plasma FSH level was increased significantly in all hypothyroid groups. The present investigation points out that suppression of T3 during the critical period of Sertoli cell proliferation affects their number and functional activity.

Molecular mechanisms of thyroid hormone-stimulated steroidogenesis in mouse leydig tumor cells. Involvement of the steroidogenic acute regulatory (StAR) protein

Using a mouse Leydig tumor cell line, we explored the mechanisms involved in thyroid hormone-induced steroidogenic acute regulatory (StAR) protein gene expression, and steroidogenesis. Triiodothyronine (T3) induced a approximately 3.6-fold increase in the steady-state level of StAR mRNA which paralleled with those of the acute steroid response ( approximately 4.0-fold), as monitored by quantitative reverse transcriptase-polymerase chain reaction assay and progesterone production, respectively. The T3-stimulated progesterone production was effectively inhibited by actinomycin-D or cycloheximide, indicating the requirement of on-going mRNA and protein synthesis. T3 displayed the highest affinity of [125I]iodo-T3 binding and was most potent in stimulating StAR mRNA expression. In accordance, T3 significantly increased testosterone production in primary cultures of adult mouse Leydig cells. The T3 and human chorionic gonadotropin (hCG) effects on StAR expression were similar in magnitude and additive. Cells expressing steroidogenic factor 1 (SF-1) showed marginal elevation of StAR expression, but coordinately increased T3-induced StAR mRNA expression and progesterone levels. In contrast, overexpression of DAX-1 markedly diminished the SF-1 mRNA expression, and concomitantly abolished T3-mediated responses. Noteworthy, T3 augmented the SF-1 mRNA expression while inhibition of the latter by DAX-1 strongly impaired T3 action. Northern hybridization analysis revealed four StAR transcripts which increased 3-6-fold following T3 stimulation. These observations clearly identified a regulatory cascade of thyroid hormone-stimulated StAR expression and steroidogenesis that provides novel insight into the importance of a thyroid-gonadal connection in the hormonal control of Leydig cell steroidogenesis.

Effects of thyroid hormone (thyroxine) and testosterone on hepatic 11beta-hydroxysteroid dehydrogenase mRNA and activity in pubertal hypothyroid male rats

To investigate the effects of thyroid hormone and testosterone on 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1), we measured changes in hepatic 11beta-dehydrogenase activity and its mRNA levels in pubertal methimazole (MMI)-induced hypothyroid male rats following treatment with thyroxine ([T4] 50 microg/kg/d) or testosterone (250 microg/d) for 14 days. Hypothyroidism in male rats markedly reduced hepatic 11beta-HSD1 mRNA levels and serum testosterone concentrations (P < .01). Subcutaneous injection of T4 in the hypothyroid rats significantly (P < .01) increased hepatic 11beta-HSD1 mRNA to approximately normal levels and simultaneously increased serum testosterone levels. However, the same daily dose of T4 administered to castrated male hypothyroid rats for 14 days did not elevate hepatic 11beta-HSD1 activity. Treatment with testosterone for 14 days in castrated hypothyroid male rats and rats without gonadectomy significantly (P < .01) increased the enzyme activity without administration of T4. Variations in hepatic 11beta-HSD1 activity were demonstrated to be accompanied by changes in serum testosterone levels in the rats following alteration of the thyroid hormone state. These results suggest that the effect of T4 in increasing the subnormal 11beta-HSD1 gene expression in hypothyroid male rats is mediated by its ability to increase testosterone production in these rats, because in castrated hypothyroid rats, T4 does not elevate 11beta-HSD1 gene expression.

Effects of 6-N-propyl-2-thiouracil on growth, hormonal profiles, carcass and reproductive traits of boars

Neonatal 6-N-propyl-2-thiouracil (PTU)-induced hypothyroidism reduces body weight but increases testicular size in adult male rodents. The objective of this study was to determine the effect of prepubertal PTU treatment on boars. For Experiment I, boars (n = 28) were randomly allotted to eight pens. Each pen received one of four PTU doses (0, 0.01, 0.03 and 0.1% in a basal diet) between 28 and 56 days of age (DOA). Due to a lack of difference among three PTU treatments, PTU-treated boars were pooled. Boars treated with PTU had lower (P < 0.05) ADG during treatment, lighter (P < 0.05) BW after 56 DOA and less (P < 0.05) developed epididymides at 154 DOA. For Experiment II, boars (n = 19) were randomly allotted to six pens. Each pen received one of three PTU treatments orally as: control (carrier), PTU-I (0.002% BW of PTU daily between 7 and 70 DOA), or PTU-II (0.002% BW of PTU daily between 28 and 91 DOA). During treatment, PTU-treated boars had lower (P < 0.05) serum T4 levels, rectal temperature, feed intake and ADG. Boars treated with PTU had lower (P < 0.05) BW between 63 and 154 DOA but higher (P < 0.05) gain/feed between 105 and 133 DOA. Boars treated with PTU had less (P < 0.05) developed epididymides and sperm count per gram testis at 238 DOA. These results suggest that prepubertal PTU-induced hypothyroidism had significant effects on growth, hormonal profiles, and reproductive traits of boars; however, it does not appear to be an effective method for increasing testis size and sperm production of commercial boars.

Euthyroid sick syndrome: an overview

Abnormalities of thyroid hormone concentrations are seen commonly in a wide variety of nonthyroidal illnesses, resulting in low triiodothyronine, total thyroxine, and thyroid stimulating hormone concentrations. These thyroid hormone changes may be mediated in part by cytokines or other inflammatory mediators, acting at the level of the hypothalamus and pituitary, the thyroid gland, and the hepatic deiodinase system, as well as on binding of thyroxine to thyroid binding globulin. The degree of thyroid function disturbance correlates with disease severity and low levels of thyroid hormones predict a poor prognosis in several illnesses. It remains unresolved whether the hormone responses in the euthyroid sick syndrome represent part of an adaptive response, which lowers tissue energy requirements in the face of systemic illness, or a maladaptive response, which induces damaging tissue hypothyroidism. Consequently, the use of thyroid hormone therapy in the euthyroid sick syndrome is controversial. The small number of controlled trials performed to date have shown conflicting results on the cardiovascular effects of triiodothyronine, and none has had the statistical power to address the question of altered mortality. Future trials of therapy should concentrate on patients with severe nonthyroidal illness and a high mortality rate. Meanwhile, better understanding is needed of the impact of the altered thyroid hormone status on tissue function.

Testicular maldescent and maldevelopment in fetal rats prenatally exposed to nitrofen

In the rat model of nitrofen-induced congenital diaphragmatic hernia, we found the testicles in a high abdominal position in many male animals, and undertook to investigate whether the teratogen interferes with testicular descent and development. Male fetuses from time-mated Wistar rats treated intragastrically with 100 mg nitrofen dissolved in oil on day 9.5 of gestation were compared with control fetuses from mothers receiving only vehicle. The litters were recovered by cesarean section on days 17, 19, and 21 of gestation; the position of the testicles in male animals was recorded, and their volume was measured prior to histological assessment of mean tubular diameter, number of germ cells per tubule, and degree of collagenization of the tunica albuginea. Testicular maldescent was present in 100% of nitrofen-exposed fetuses on the 17th gestational day, 77% of those recovered on day 19, and 41% of those near term (21st day), whereas all control animals but 1 had "descended" gonads on all three days. Testicular volume was significantly decreased in treated fetuses on the 21st gestational day, and the mean tubular diameter was significantly decreased in all three age groups. Experimental and control animals had similar numbers of germ cells per tubule. The albuginea layer had apparently normal collagen content in all groups. These findings suggest that prenatal exposure to nitrofen interferes with both transabdominal descent of the testicle (transinguinal descent is postnatal in the rodent) and its normal development. Previous evidence and the present results authorize speculation on the possible role of nitrofen-induced prenatal thyroid hypofunction in the pathogenesis of maldescent and maldevelopment in this model, since thyroid hormones act directly on Sertoli cells, which secrete müllerian inhibiting substance, which is likely responsible for transabdominal descent.

Enhancement of seminiferous tubular growth and spermatogenesis in testes of rats recovering from early hypothyroidism: a quantitative study

Testicular weight and DNA content were markedly reduced (63 and 69%) in weanling Long-Evans rat pups rendered hypothyroid from birth by administration of propylthiouracil (PTU), a reversible goitrogen. These growth deficits worsened to > 80% by continuing hypothyroidism beyond weaning, to days 50 and 90. Recovery of thyroid function, brought about by discontinuing PTU at weaning, resulted in a paradoxical stimulation of testis growth, amounting to increased weight (40%), DNA content (60%) and size by 90 days, compared to age-matched controls. In the 25-day or older hypothyroid rats, testicular structure was immature and spermatogenesis markedly delayed, as evident by closed lumen and significantly reduced diameter of seminiferous tubules (38%), thickness of germinal layer (70%), and number of primary spermatocytes (86%), compared to control. Hypothyroidism did not alter the number of tubules per testis cross section. In the 90-day recovery rats, numbers of seminiferous tubules were unchanged but tubular diameter was significantly (20%) larger than in controls and spermatogenesis appeared very active as indicated by significantly increased germinal layer thickness (22%) and total number and density of primary spermatocytes (55% and 40%). The results show that although postnatal hypothyroidism is deleterious for testicular growth and spermatogenesis, recovery from this condition leads to enhanced seminiferous tubular growth and spermatogenesis.