Effects of controlled ovarian stimulation on thyroid function during pregnancy

Controlled ovarian stimulation (COS) is a major component of assisted reproductive technologies. Clinically, it has been observed that, some women experience changes in thyrotropin levels following COS, which then bring about subclinical hypothyroidism and may adversely affect conception. Studies have also shown that the specific degree as well as the tendency of changes in thyroid function vary with differences in thyroid function before pregnancy, thyroid autoimmunity, the COS regimen, and the observation time point. However, the associated pathophysiological mechanism of the effects of COS on pregnancy has not yet been fully elucidated. This may be because increased estradiol levels, caused by COS, induce increased levels of thyroxine-binding globulin, resulting in a decrease in free thyroxine (FT4) level and an increase in TSH level. Conversely, it has also been observed that human chorionic gonadotropin (hCG) can act directly on thyroid cells, exerting opposite effects on FT4 and TSH levels. Additionally, the effects of COS on thyroid function may be more pronounced, especially in women with autoimmune thyroid disease or thyroid dysfunction before pregnancy, ultimately leading to subclinical hypothyroidism. Here, we review recent research progress regarding the effects of COS on thyroid function during pregnancy.

Changes in thyroid function during controlled ovarian hyperstimulation (COH) and its impact on assisted reproduction technology (ART) outcomes: a systematic review and meta-analysis

BACKGROUND

To investigate the thyroid function changes during controlled ovarian hyperstimulation (COH) and ascertain its impact on reproductive outcomes.

METHODS

We conducted meta-analysis in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A comprehensive literature search was performed to identify studies reported changes in thyroid parameters during COH. We analyzed thyroid-stimulating hormone (TSH) levels, free thyroxin (fT4) levels, changes in estrogens (E2), thyroxine-binding globulin (TBG), relative risks (RRs) of clinical pregnancy rate (CPR), live birth rate (LBR), and mean difference (MD) of TSH increment between the miscarriage group and ongoing pregnancy group.

RESULTS

This meta-analysis included fifteen individual studies (n = 1665 subjects). At the end of COH, the mean TSH (2.53 mIU/L; 95% CI, 2.19 to 2.88; I² = 92.9%) exceeded the upper limit (2.5 mIU/L) and remained above the threshold until one month following embryo transfer (ET). Thyroxin decreased from baseline to the end of COH (-0.18 ng/l; 95% CI, -0.35 to 0.00; I² = 92.2%). The CPR and LBR of patients with TSH exceeding the cutoff after COH were significantly lower than those of patients with TSH below the threshold (CPR: RR, 0.62; 95% CI, 0.47 to 0.82; I² = 0.0% and LBR: RR, 0.64; 95% CI, 0.44 to 0.92; I² = 0.0%). The MD of the increment in TSH levels between the miscarriage and ongoing pregnancy groups was 0.40 mIU/L (95% CI, 0.15 to 0.65; I² = 0.0%).

CONCLUSIONS

This meta-analysis shows that TSH increases and fT4 decreases during COH. COH-induced thyroid disorder impairs reproductive outcomes.

Thyroid function modifications in women undergoing controlled ovarian hyperstimulation for in vitro fertilization: a systematic review and meta-analysis

OBJECTIVE

To investigate the impact of controlled ovarian hyperstimulation (COH) for in vitro fertilization (IVF) on thyroid function.

DESIGN

Systematic review and meta-analysis.

SETTING

Not applicable.

PATIENT(S)

Infertile women undergoing conventional IVF or intracytoplasmic sperm injection.

INTERVENTION(S)

Systematic search of PubMed, MEDLINE, Embase, Scopus, and Web of Science from inception until November 2020. Studies could be included only if they met the following criteria: subjects were classified as euthyroid or hypothyroid; serum thyroid-stimulating hormone (TSH) and/or free thyroxine (FT4) levels were evaluated before COH; and the same thyroid function test was reassessed after COH (i.e., at the time of trigger for final follicle maturation and/or at pregnancy test).

MAIN OUTCOME MEASURE(S)

Mean difference (MD) between the serum TSH or FT4 levels assessed after COH and before COH.

RESULT(S)

In euthyroid women, the serum TSH levels assessed at the time of trigger and at the time of pregnancy test were significantly higher than those at baseline (MD: 0.69 mIU/L, 95% confidence interval [CI]: 0.30-1.08, I² = 93% and MD: 0.67, 95% CI: 0.49-0.85, I² = 72%, respectively). The serum FT4 levels did not undergo significant changes. Subanalysis confirmed an increase in the TSH level after restricting the analysis to women treated with gonadotropin-releasing hormone agonist protocols and to those who achieved pregnancy. A pronounced increase in the TSH level was observed in women treated for hypothyroidism (MD: 1.50 mIU/L, 95% CI: 1.10-1.89, I² = 0%).

CONCLUSION(S)

Pooling of the results showed a significant increase in serum TSH level in women undergoing COH for IVF. This change was particularly pronounced in women treated for hypothyroidism. New thyroid function screening strategies for women undergoing COH are warranted.

In vitro fertilization/intracytoplasmic insemination and thyroid function: reviewing the evidence

Recent findings, that specific G protein-coupled TSH receptors (TSHR) and the nuclear thyroid hormone receptors (THRs) are widely expressed in reproductive tissues, reveal the close links between hypothalamus-pituitary-thyroid and hypothalamus-pituitary-gonadal axes. It has been suggested that thyroid function as well as thyroid autoimmunity (TAI) have an impact on Assisted Reproduction Technology (ART) reproductive outcome. Lately, it became evident that ovarian stimulation (OS) may also have an impact on thyroid function. This narrative review describes the mutual interaction between thyroid function and OS, by reviewing the current evidence, assessing the pathophysiological links and arriving at practical recommendations.