Thyroid-Stimulating Hormone (TSH)
Thyroid-Stimulating Hormone (TSH) is a hormone produced by the pituitary gland at the base of the brain. It regulates how much hormone the thyroid produces. The thyroid produces two of these hormones: thyroxine (T4) and triiodothyronine (T3). When the thyroid slows down (hypothyroidism), TSH rises. When the thyroid produces too much hormone (hyperthyroidism), TSH falls. TSH is the first blood test used to screen for thyroid dysfunction.
Most labs define a normal TSH as about 0.5 to 4.5 mIU/L. A normal value is not the same as an optimal value for conception. One study examined 239 women treated at a large hospital system. Every participant had a TSH within the lab's normal range. In that group, 187 women had unexplained infertility. The other 52 women had no fertility problem of their own. Their infertility came from a partner with very low or absent sperm counts (severe oligospermia or azoospermia). The women with unexplained infertility had higher TSH than this comparison group. Their median TSH value was 1.95 mIU/L, compared with 1.66 mIU/L in the comparison group.1 This difference has clinical relevance. A general practitioner may not regard a TSH of 3.8 mIU/L as a concern. A restorative clinician reads the same value as a possible cause of cycle disorders.
Many restorative clinicians aim for thyroid function that is optimal for conception, not only within the lab's normal range. The concern is that low-normal thyroid function can affect ovulation, the luteal phase, and early pregnancy.
TSH alone does not provide the complete picture. A thorough thyroid evaluation also measures free T4, free T3, and thyroid antibodies. A 2019 Cochrane review studied thyroid hormone replacement in women with subfertility, or difficulty conceiving. In women who had thyroid antibodies but normal thyroid function, live birth rates were about the same with or without treatment (RR 1.04, 95% CI 0.83 to 1.29). Some women had a mildly underactive thyroid detected only on blood tests (subclinical hypothyroidism). In these women, live birth rates were higher with treatment (RR 2.13, 95% CI 1.07 to 4.21).2 The authors of the review rated this evidence as low to very low quality. A small, low-quality set of trials that finds no difference does not prove the absence of benefit. Thyroid antibodies can still harm fertility and raise the risk of miscarriage for reasons unrelated to the TSH value.
Cited in this entry
- Orouji Jokar T et al. Higher TSH Levels Within the Normal Range Are Associated With Unexplained Infertility. J Clin Endocrinol Metab. 2017. https://rrmacademy.org/library/higher-tsh-levels-within-the-normal-range-are-associated-with-unexplained-infert-recfswcfajl43swbh/
- Akhtar MA et al. Thyroxine replacement for subfertile women with euthyroid autoimmune thyroid disease or subclinical hypothyroidism. Cochrane Database Syst Rev. 2019. https://rrmacademy.org/library/thyroxine-replacement-for-subfertile-women-with-euthyroid-autoimmune-thyroid-dis-recibmgwj63qmhtxs/
This content is for educational purposes only and does not constitute medical advice. Consult an RRM clinician or healthcare provider for guidance specific to your situation.
