Thyroid-testis interrelationship during the development and sexual maturity of the rat

Thyroid impairment and male fertility: a narrative review of literature

PURPOSE

To evaluate the effect of thyroid function on male fertility, focusing on hypo- and hyperthyroidism.

METHODS

A PubMed/MEDLINE, Web of Science, and Scopus research was performed. Original studies in English published online up to 31 May 2023 were selected and reviewed. The final reference list was defined based on the relevance of each paper to the scope of this review.

RESULTS

The available data in animals (31 studies) and human (26 studies) showed conflicting results. However, thyroid dysfunction altered erection and ejaculation both in animal models than in men.

CONCLUSION

Both hypothyroidism and hyperthyroidism seem to cause ejaculation and erectile dysfunction. Hence, Guidelines recommend against the systematic screening for thyroid disorders in the men in sub-fertile couples, but only in men with ejaculation and erectile dysfunction and/or altered semen parameters.

Imbalanced testicular metabolism induced by thyroid disorders: New evidences from quantitative proteome

Thyroid dysfunctions, such as hypothyroidism and hyperthyroidism, are the second most prevalent endocrinopathies and are associated to reproductive disorders in men. Several genes are differentially modulated by thyroid hormones in testes and imbalances in thyroid hormone levels are also associated to alterations on sperm functionality. Imbalances on antioxidant defense mechanism and stress oxidative have been pointed out as the main factors for the impairments on male reproductive function. To clarify this issue, we investigated the expression and activity of antioxidant enzymes in testis, followed by their proteomic profile in attempt to characterize the mechanisms involved in the alterations induced by hypo- or hyperthyroidism in adult male rats. Hypothyroidism reduced the Gsr transcript expression and the activity of CAT and GSR enzymes, while the hyperthyroidism reduced the Gpx4 var2 transcript expression. Among 1082 identified proteins, 123 and 37 proteins were downregulated by hypothyroidism compared to euthyroid and hyperthyroid condition, respectively, being 36 proteins commonly reduced in both comparisons and one exclusively in hypo-hyperthyroidism comparison. A network containing 29 nodes and 68 edges was obtained in protein-protein interaction analysis and the functional enrichment analysis of differentially expressed proteins revealed significant alterations for several functions in hypo-euthyroid and hypo-hyperthyroid comparisons, such as ATP metabolic process, coenzyme binding, sperm part, peroxiredoxin activity, mitochondrial protein complex, intramolecular oxidoreductase activity, binding of sperm to zona pellucida, glutathione transferase activity, response to testosterone. Thus, there is a correlation between thyroid disorders and impaired antioxidant defense mechanism, resulting in reproductive dysfunctions, as infertility, mainly observed in hypothyroidism.

Comparative Aspects of Pre- and Postnatal Development of the Male Reproductive System

This review describes pre- and postnatal development of the male reproductive system in humans and laboratory animals, and highlights species differences in the timing and control of hormonal and morphologic events. Major differences are that the fetal testis is dependent on gonadotropins in humans, but is independent of such in rats; humans have an extended postnatal quiescent period, whereas rats exhibit no quiescence; and events such as secretion by the prostate and seminal vesicles, testicular descent, and the appearance of spermatogonia are all prenatal events in humans, but are postnatal events in rats. Major differences in the timing of the developmental sequence between rats and humans include: gonocyte transformation period (rat: postnatal day 0-9; human: includes gestational week 22 to 9 months of age); masculinization programming window (rat: gestational day 15.5-17.5; human: gestational week 9-14); and mini-puberty (rat: 0-6 hr after birth; human: 3-6 months of age). Endocrine disruptors can cause unique lesions in the prenatal and early postnatal testis; therefore, it is important to consider the differences in the timing of the developmental sequence when designing preclinical studies as identification of windows of sensitivity for endocrine disruption or toxicants will aid in interpretation of results and provide clues to a mode of action. Birth Defects Research 110:190-227, 2018. © 2017 Wiley Periodicals, Inc.

New insights for male infertility revealed by alterations in spermatic function and differential testicular expression of thyroid-related genes

The impact of thyroid hormone (TH) disorders on male reproductive biology has been a controversial issue for many years. Recently, we reported that hypothyroid male rats have a disruption of the seminiferous epithelium, which may compromise spermatogenesis. To improve the understanding of the reproductive pathogenesis of hypothyroidism and hyperthyroidism, male Wistar rats that developed these dysfunctions in adulthood were used as an experimental model. We evaluated the sperm production, reserves, transit time, morphology, and functionality (acrosome integrity, plasma membrane integrity, and mitochondrial activity), and the testicular expression of the TH receptors (Thra1 and Thra2, Thrb1, and Thrb2), deiodinases (Dio2 and Dio3), and the Mct8 transporter (Slc16a2) were assessed by reverse transcription followed by real-time quantitative PCR (RT-qPCR). The results were evaluated statistically by ANOVA and Tukey HSD test (P < 0.05). Hypothyroidism decreased the total and daily sperm productions and increased the sperm transit time through the epididymis, while the sperm functionality was reduced in both thyroid dysfunctions. Regarding the modulation of gene expression in the testis, hypothyroidism increased the expression of Thra1 and decreased the expression of Dio3, and hyperthyroidism increased the expression of Slc16a2. The observed alterations in spermatic production and function and in the expression of the TH receptor, deiodinase, and the TH transporter are suggestive of TH participation in spermatogenesis in adulthood.

In vivo models for male reproductive toxicology

In Vivo Models for Male Reproductive Toxicology (Rochelle W. Tyl, Research Triangle Institute, Research Triangle Park, North Carolina). Assessment of male reproductive function requires a specific set of evaluations of the various steps in successful mating from sperm production to copulation to fertilization to production of a viable litter. This unit outlines the measurements that are standard for determining the effects of treatment with toxicant on the reproductive capacity of male mice and rats.

Compromised Rat Testicular Antioxidant Defence System by Hypothyroidism before Puberty

Altered thyroid function during early stages of development is known to affect adversely testicular growth, physiology, and antioxidant defence status at adulthood. The objective of the present study is to investigate the modulation of antioxidant defence status in neonatal persistent hypothyroid rats before their sexual maturation and also to identify the specific testicular cell populations vulnerable to degeneration during neonatal hypothyroidism in immature rats. Hypothyroidism was induced in neonates by feeding the lactating mother with 0.05% 6-n-propyl-2-thiouracil (PTU) through the drinking water. From the day of parturition till weaning (25 day postpartum), the pups received PTU through mother's milk (or) drinking water and then directly from drinking water containing PTU for the remaining period of experimentation. On the 31st day postpartum, the animals were sacrificed for the study. An altered antioxidant defence system marked by elevated SOD, CAT, and GR activities, with decreased GPx and GST activities were observed along with increased protein carbonylation, disturbed redox status in hypothyroid immature rat testis. This compromised testicular antioxidant status might have contributed to poor growth and development by affecting the spermatogenesis and steroidogenesis in rats before puberty as indicated by reduced germ cell number, complete absence of round spermatids, decreased seminiferous tubule diameter, and decreased testosterone level.

Severe and mild neonatal hypothyroidism mediate opposite effects on Leydig cells of rats

The effects of neonatal hypothyroidism on the number of Leydig cells were studied in neonatal Wistar rats. Moderate or severe hypothyroidism were induced during neonatal life by giving different amounts of methimazole (MMI; 0.05% or 0.1%) in the drinking water of pregnant and lactating dams. Rats were sacrificed on day 21 of postnatal life. Severely hypothyroid rats had approximately 45-fold higher serum thyrotropin (TSH) values and demonstrated approximately a 65% decrease in testes weight (p < 0.05) and the number of Leydig cells. However, in moderately hypothyroid rats, in which serum TSH was only approximately 16-fold higher, testicular weight was normal and the number of Leydig cells almost doubled (p < 0.01). There were no significant differences between the serum-free testosterone levels of the moderately and severely hypothyroid rats versus controls. Serum levels of 3alpha-androstanediol glucuronide, although decreased to less than 10% in severely hypothyroid rats (p < 0.01), were not changed by mild hypothyroidism. The number of Sertoli cells was increased in moderately hypothyroid rats versus controls (p < 0.05) and even further increased in severely hypothyroid rats (p < 0.05). We conclude that (1) severe neonatal hypothyroidism impairs the development and function of the testes and (2) moderate neonatal hypothyroidism stimulates the proliferation of Leydig cells.

Characteristics of infertility and the improvement of fertility by thyroxine treatment in adult male hypothyroid rdw rats

We previously reported that rdw rats were infertile in both sexes. The present study was conducted to determine whether hypothyroidism in adult male rdw rats induced infertility by impairing sexual behavior or testicular function, whether the infertility could be reversed by thyroxine (T(4)) treatment, and whether the mutant could be produced by infertile rdw rats via in vitro fertilization. The sexual behavior was analyzed by pairing with normal female rats. The fertility of epididymal sperm was determined by in vitro fertilization. The results indicated that the infertility resulted from both defective sexual behavior and testicular function. No untreated rdw rats mated. The weights of epididymides were significantly low, whereas those of testes were not different from those of untreated normal rats. Epididymal sperm with cytoplasmic droplets were observed at a significantly high frequency. No fertilization was detected either in vivo or in vitro. Thyroxine treatment markedly increased serum T(4) levels and the weights of both epididymides and testes. Partial reversion of the impaired sexual behavior was observed, and the percentage of epididymal sperm with cytoplasmic droplets was markedly decreased after T(4) treatment. Fertility of epididymal sperm was completely reversed when determined both in vivo and in vitro, and homozygous embryos developed to term after transfer without loss of viability.

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.

Effect of hypothyroidism induced by propylthiouracil and thiourea on male and female reproductive systems of neonatal mice

The effect of hypothyroidism induced by 6-propyl-2-thiouracil (PTU) or thiourea (TU) on the development of the reproductive system in male and female neonatal ICR mice was investigated. PTU or TU was injected subcutaneously into experimental animals from postnatal day 1 (PD1) onward. The histological changes of the reproductive organs, formation of ovarian follicles, and spermatogenesis were examined on PD 14, 21, and 28, and the fertility of the hypothyroid mice in adulthood was followed. It was found that PTU or TU treatment did not produce an effect on the histology of the neonatal uterus and oviduct. In contrast, the drugs induced a decrease in the number of primordial follicles, multilaminar follicles, and Graafian follicles in the ovary. The number of follicles with degenerated follicular cells was increased. In the testis both PTU and TU treatments brought about a decrease in the number of seminiferous tubules with developing spermatids although the mean diameter of seminiferous tubules and the histology of the testis, epididymis, seminal vesicle, and coagulating gland was unaffected. The mating between hypothyroid females and euthyroid males and that between hypothyroid males and euthyroid females were normal with regard to the pregnancy rate, litter size, and sex ratio of offspring. The somatic growth of the resulting offspring was normal. It is concluded that the retarding effect on ovarian and testicular development in mice during neonatal period was not serious enough to adversely affect reproduction in the hypothyroid animals.

Reproductive manifestations of thyroid disease

Thyroid function and reproductive function have many interactions, the scope and mechanism of which are not fully understood. These functions are of greatest clinical importance for veterinarians working with breeders of purebred dogs. Thyroid dysfunction does not always result in clinical signs of reproductive disorders or in subfertility. It seems that animals with overt thyroid dysfunction are those most likely to manifest reproduction problems.

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.

Thyroid hormones and testis development: a model system for increasing testis growth and sperm production

In conclusion, the unique transient hypothyroidism system described here provides a noninvasive means by which testicular size and sperm production can be increased to unprecedented degrees. This system may be useful as a model for understanding the factors which regulate testicular growth and the eventual size that the testes attain, as well as the factors which normally establish and then maintain the amount of sperm produced by an animal.