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7.1 Fertility
Hyperthyroidism affects the fertility and overall reproduction of species by influencing development of the reproductive tract. In humans, thyroid plays a role in fertility by acting on gametes during the fertilization process. In doing so, elevated thyroid levels can even lead to temporary infertility, which is defined as the inability to conceive after regularly unprotected intercourse." Unlike the association between hypothyroidism and fertility, the connection between hyperthyroidism and fertility is relatively weak and the effects are often reversible.

Particularly in women, hyperthyroidism can lead to hormonal imbalance that may impact menstrual cycles. This is common in women with elevated thyroid levels, but may not be the cause of infertility as these women continue to ovulate. Fertility in women with elevated thyroid hormone may also be impacted by symptoms such as endometriosis, ovarian failure and Polycystic ovary syndrome. In females, increased thyroid levels may lead to increases in other hormone levels such as those of estrogen, rostenedione, and testosterone. Unlike in males, hyperthyroidism in pre-pubescent ages in females does not appear to affect reproductive development.

In men, the effects of hyperthyroidism are more subtle than those of hypothyroidism. However, symptoms in males with hyperthyroidism account for 30% of couple infertility Although symptoms dissipate as hormone levels are restored, thyroid level imbalance in pre-pubescent ages can disrupt the reproductive system development of males which can later impact fertility. Specifically, hyperthyroidism may cause disruption of the hypothalamic pituitary gonadal axis which can lead to temporary sperm abnormalities such as reduction in sperm motility, decreased libido, testes enlargement, and changes in sperm morphology and density. Hyperthyroidism in males may also lead to infertility via symptoms such as erectile dysfunction, premature ejaculation, and compromised testicular activity due the oxidative stress placed on the testes. The impact of high thyroid levels on fertility is not permanent as thyroid imbalance in humans is manageable, and its effects are reversible upon hormone level restoration. Restoring thyroid levels to a balanced condition, known as euthyroidism, commonly corrects the main causes of infertility, such as menstrual disturbances and seminal parameters. However, individuals who have suffered from thyroid imbalance long-term or during pre-pubescent ages may experience permanent abnormalities in reproductive function.

In many individuals, hyperthyroidism may not influence fertility, but euthyroid states are nevertheless recommended. It is possible to test whether hyperthyroidism is leading to infertility using lab work and assessing the medical history of both parents-to-be. This can be done during a pre-conception counseling appointment. If hyperthyroidism is influencing fertility, available treatment options include: anti-thyroid medications, radioactive iodine (RAI), and thyroidectomy. These options are recommended prior to conception and require patient-physician consultation to be administered safely. RAI is not known to cause permanent effects on human gametes but avoiding conception for 6 months is recommended. RAI administration is sometimes avoided in women seeking fertility who do not use contraception. Regardless of treatment choice, the use of contraception is recommended to all couples experiencing infertility due to hyperthyroidism until the completion of treatment. In males, radioactive iodine treatment may lead to reductions in sperm count and motility, but the permanent risk of these effects remain small.