Thyroid dysfunction is one of the most common and most treatable causes of infertility in both men and women, yet it is routinely missed in standard fertility workups that focus on sex hormones, ovarian reserve, and structural anatomy. The thyroid regulates every aspect of reproductive function — from ovulation to implantation to semen quality — and correcting thyroid dysfunction often resolves infertility that other interventions could not.
Thyroid Hormone and the HPO Axis
The hypothalamic-pituitary-ovarian (HPO) axis — the hormonal system that regulates ovulation — is exquisitely sensitive to thyroid status. Both hypothyroidism and hyperthyroidism disrupt the normal pulsatile pattern of GnRH (gonadotropin-releasing hormone) from the hypothalamus, which in turn disrupts the pattern of LH and FSH secretion from the pituitary. This produces anovulatory cycles — menstrual cycles in which ovulation does not occur — even when sex hormone levels appear otherwise normal. The disruption of the GnRH pulse frequency is one of the primary mechanisms by which thyroid dysfunction causes infertility.
In hypothyroidism, elevated TRH (thyrotropin-releasing hormone) from the hypothalamus also stimulates prolactin secretion from the pituitary — a condition called hyperprolactinemia, which further suppresses GnRH pulsatility and ovulation. This is why many women with subclinical hypothyroidism develop elevated prolactin and anovulatory infertility, and why correcting the thyroid disorder often resolves both the elevated prolactin and the anovulation simultaneously.
Hashimoto’s and Pregnancy Loss
Hashimoto’s thyroiditis — autoimmune destruction of the thyroid — is the most common cause of hypothyroidism in developed countries, and it carries specific risks for pregnancy beyond those associated with hypothyroidism alone. The autoimmune process itself appears to have direct effects on the endometrium and placental function, increasing the risk of miscarriage even in women whose thyroid hormone levels are well-controlled with levothyroxine. The proposed mechanism involves thyroid peroxidase antibodies cross-reacting with similar enzymes in the endometrium, and the general pro-inflammatory state associated with autoimmune disease affecting implantation and placental development.
For women with Hashimoto’s and a history of recurrent pregnancy loss, the evidence supports treating to a TSH target of below 2.5 mIU/L (and preferably below 1.5-2.0) in the preconception period. Some reproductive endocrinologists also recommend selenium supplementation (200mcg daily as selenomethionine) for Hashimoto’s patients, given the evidence that selenium reduces anti-TPO antibody titres and may reduce the autoimmune component of the condition.
Subclinical Hypothyroidism and Male Fertility
Thyroid dysfunction affects male fertility as well, though the mechanism is different. Hypothyroidism in men reduces sex hormone-binding globulin (SHBG) production, disrupting the ratio of free testosterone to total testosterone, and producing a relative testosterone deficiency pattern. More directly, thyroid hormone is required for normal sperm production — hypothyroidism is associated with reduced sperm count, reduced sperm motility, and increased abnormal sperm morphology. These effects are reversed by thyroid hormone replacement, and they illustrate that male infertility evaluation should include thyroid function testing.
Practical Preconception Thyroid Management
The standard preconception thyroid evaluation should include TSH, free T4, free T3, reverse T3, and anti-thyroid peroxidase (anti-TPO) antibodies. TSH should be below 2.5 mIU/L before attempting conception. If TSH is elevated with normal free T4, this is subclinical hypothyroidism — it still impairs fertility and should be treated with low-dose levothyroxine. If anti-TPO antibodies are elevated (indicating Hashimoto’s), selenium supplementation at 200mcg daily is indicated alongside thyroid hormone replacement if TSH is also elevated.
For men with low-normal thyroid function and suboptimal semen parameters, a trial of thyroid hormone (under medical supervision) can improve semen quality within 3-6 months. The dose used is typically low — 25-50mcg of levothyroxine — and the effect is monitored via repeat semen analysis and thyroid labs. The combination of thyroid support and the mitochondrial support protocol described previously (CoQ10, L-carnitine, zinc, selenium) is a reasonable comprehensive approach to male factor infertility with thyroid involvement.
Selenomethionine vs Sodium Selenite
The two most common supplemental forms are selenomethionine (organic, from selenised yeast) and sodium selenite (inorganic). Selenomethionine is absorbed via methionine transport pathways and incorporated into body protein stores, providing a slow-release mechanism. Comparative trials show selenomethionine raises plasma selenium more effectively than sodium selenite at equivalent doses.
Why Selenium Status Matters for Thyroid Patients
Selenium deficiency impairs both antioxidant defence and thyroid hormone activation. In people with Hashimoto thyroiditis, selenium supplementation at 200mcg per day has been shown to reduce anti-TPO antibodies by approximately 40% in randomised controlled trials, with concurrent reductions in thyroiditis activity. The mechanism involves reduced oxidative stress in the thyroid gland and modulation of the autoimmune response.
Selenomethionine vs Sodium Selenite
The two most common supplemental forms are selenomethionine (organic, from selenised yeast) and sodium selenite (inorganic). Selenomethionine is absorbed via methionine transport pathways and incorporated into body protein stores, providing a slow-release mechanism. Comparative trials show selenomethionine raises plasma selenium more effectively than sodium selenite at equivalent doses.
Why Selenium Status Matters for Thyroid Patients
Selenium deficiency impairs both antioxidant defence and thyroid hormone activation. In people with Hashimoto thyroiditis, selenium supplementation at 200mcg per day has been shown to reduce anti-TPO antibodies by approximately 40% in randomised controlled trials, with concurrent reductions in thyroiditis activity. The mechanism involves reduced oxidative stress in the thyroid gland and modulation of the autoimmune response.
A quality supplement routine can make a real difference to your results.
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