Link Between Ovarian Reserve and Thyroid Stimulating Hormone Level Observed

Increased levels of thyroid stimulating hormone (TSH) and aged >35 years correspond to decreased ovarian function as measured by Anti-Mullerian hormone (AMH) levels, according to a study published in Thyroid Research.

Researchers enrolled 314 women referred to the Al-Zahra Hospital in Rasht, Iran between 2019 and 2020 in a cross-sectional, prospective study. Of these women, 172 had AMH levels ≥ 1.1 ng/ml and 142 had AMH levels < 1.1 ng/ml. Women were further divided into two groups: 124 women aged < 35 years and 190 women aged ≥35 years. Women with iatrogenic causes of decreased ovarian reserve, such as prior ovarian surgery, radiotherapy, chemotherapy, endometriosis, or a history of known thyroid disorders, levothyroxine users, and polycystic ovarian syndrome (PCOS) were excluded.

Approximately 70% of the women >35 years and 19.4% of women ≤35 had AMH levels under 1.1 ng/ml. In women >35 years, median TSH levels with AMH <1.1 ng/ml were significantly higher than those with AMH ≥ 1.1 ng/ml (P = .037).

The researchers noted a relationship between TSH and AMH, reporting that, for every one unit increase in TSH level, the probability of a woman having AMH levels ˂ 1.1 ng/ml increased by 25% (P =.017). In women aged <35 years, a TSH level of 1.465 mIU/L denoted the cut-off point where decreased AMH levels (80.5% sensitivity, 38.6% specificity, area under the curve [AUC] =.596) were identified.

In addition to TSH and AMH, other measurements included FSH and estradiol (E2) on days 2-4 of the menstrual cycle, thyroxine (free T4), luteinizing hormone (LH), and body mass index (BMI). There was no significant difference in BMI in the aged >35- and <35- year-old groups based on AMH levels (P =.102 and P =.909, respectively).

The authors said the inability to evaluate antithyroid antibodies and the small sample size of the study group may have limited their conclusions. Another limitation was the researchers’ selection of participants from a population who had a history of fertility dysfunction (inability to become pregnant after more than one year of unprotected sex) which may not be generalizable to all women. Larger prospective studies are needed.

The authors commented that their results reflected the findings of other studies which imply “aging is associated with a decrease in the quality and quantity of ovarian function.” They acknowledged serum TSH levels less than 3 mIU/ml in previous studies were associated with better ovarian function; however, if TSH was higher than this cut-off point, treatment with levothyroxine was recommended. Another study (Kuroda et al, 2018) showed levothyroxine therapy “failed to change ovarian reserve in clinical and subclinical hypothyroidism, even though AMH level was significantly increased in patients with Hashimoto thyroiditis.”

“Our study supports the inverse relationship between thyroid-stimulating hormone and ovarian reserve, and an increase in TSH from a cut-off point of 1.465 mIU/L in participants over 35 is associated with a decrease in ovarian function,” concluded the researchers.

Reference

Kabodmehri R, Sharami SH, Sorouri ZZ, et al. The relationship between thyroid function and ovarian reserve: a prospective cross-sectional study. Thyroid Res. 2021;14:22. doi:10.1186/s13044-021-00112-2