Thyroid Function and Fertility
Hashimoto’s thyroiditis is one of the most commonly encountered hormonal abnormalities and is a frequent cause of subclinical hypothyroidism (decreased production of thyroid hormones with no detectable symptoms). Hashimoto’s thyroiditis is associated with recurrent miscarriage, unexplained infertility, repeated in vitro fertilization failure, and clomid failure in patients with polycystic ovarian syndrome (PCOS). Women with Hashimoto’s also experience increased preterm labor and hypertensive disorders of pregnancy. Even subclinical hypothyroidism (TSH > 2.5 uIU/ml, it is still associated with the problems described above.
A recent paper out of Austria (Reproductive Biology and Endocrinology 2014, 12:28), examined the impact of thyroid function on intrauterine insemination success. They looked at 540 patients who had undergone intrauterine insemination with the primary outcome parameter being whether or not they achieved a clinical pregnancy. They analyzed several parameters relating to thyroid disease including whether they had subclinical hypothyroidism, overt hypothyroidism, and if the woman was under treatment with thyroid hormone supplementation.
In this study, patients with subclinical hypothyroidism who were treated with thyroid hormone supplementation had higher pregnancy rates (23.9%) than patients who had no thyroid medication (5.1%). The TSH (thyroid stimulating hormone) threshold for diagnosing subclinical hypothyroidism in this study was greater than 2.5 uIU/ml and less than 5uIU/ml. All of the patients who were treated with thyroid hormone supplementation maintained TSH levels below 2.5 uIU/ml indicating satisfactory thyroid hormone supplementation.
Although “overt hypothyroidism” is clearly associated with poorer reproductive outcomes, in this study when TSH levels were maintained below 2.5uIU/ml, there was no reduction in pregnancy rates among patients undergoing intrauterine insemination. Treating subclinical hypothyroidism may have a favorable impact on other forms of infertility treatment such as in vitro fertilization and additional studies need to be done to address this issue. Future studies need to address the possibility that there is an ideal TSH level that optimizes treatment success. For example, would the patients in this study have had higher pregnancy rates had their TSH levels been suppressed below 2.0 or 1.5 uIU/ml? Although most infertility evaluations include a general assessment of thyroid function, it is intriguing that a more precise approach that targets a specific TSH might optimize success rates of various infertility treatments. Prospective studies are needed to determine ideal thyroid hormone levels for maximizing infertility treatment success. It seems prudent to evaluate thyroid function in all infertility patients and carefully treat subclinical and overt hypothyroidism. Once thyroid supplementation is initiated, suppression of TSH levels to below 2.5 uIU/ml should be the goal of treatment.