What affects functional ovarian reserve, thyroid function or thyroid autoimmunity?

Background: Thyroid dysfunction is the most common autoimmune endocrine disorder in women of reproductive age, and is associated with menstrual irregularities, anovulation and infertility. Whether it is thyroid function or thyroid autoimmunity that affects functional ovarian reserve (FOR, i.e., the small growing ovarian follicle pool) reflected in anti-Müllerian hormone (AMH) has, however, remained under dispute. Methods: We investigated in 225 infertile women whether thyroid function, after adjustment for thyroid autoimmunity, affects FOR within what is considered normal thyroid function (TSH, 0.4-4.5μIU/mL) by assessing AMH levels in reference to TSH levels, stratified for TSH < or ≥3.0μIU/mL. Thyroid autoimmunity was defined by presence of anti-thyroid peroxidase, -thyroglobulin and/or -thyroid receptor antibodies. Results: Mean age of studied women was 38.4 ± 5.0years; their mean AMH was 1.3 ± 2.0ng/mL and mean TSH 1.8 ± 0.9 μIU/mL. Thyroid autoimmunity was present in 11.1% of patients. Women with TSH <3.0μIU/mL presented with significantly higher AMH compared to those with TSH ≥3.0μIU/Ml (P = 0.03). This difference remained significant after adjustment for thyroid autoimmunity as well as age (P = 0.02). Conclusions: Even after adjustment for thyroid autoimmunity and age, TSH <3.0μIU/mL in euthyroid infertility patients is associated with significantly better FOR (higher AMH) than TSH ≥3.0μIU/mL. This observation suggests a direct beneficial effect of lower TSH levels on follicular recruitment, and warrants investigations of thyroxin supplementation in infertile women with TSH levels ≥3.0μIU/mL in attempts to improve FOR.