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29 January 2018.

Infertility is not uncommon. About 15% of couples in the UK seek advice at some time about difficulty in achieving pregnancy. Infertility is termed idiopathic or unexplained when conception has not occurred after more than a year of regular unprotected intercourse every 2 to 3 days, the woman has no hormonal disorder or infection, has had normal menstrual cycles with normal fallopian tubes and uterus, and the man has a normal semen analysis. Up to aquarter of those seeking help will have unexplained infertility.

Hypothyroidism is a known cause of infertility. When thyroid hormone production falls, the pituitary gland produces thyroid stimulating hormone (TSH) to increase production of thyroxine (T4) and tri-iodothyronine (T3). If this is not fully successful and thyroid function remains low, signs and symptoms of hypothyroidism, including infertility, result. These can be reversed by treatment with tablets of levothyroxine.

Pregnancy rates are also low even if failing thyroid hormone production is increased to normal by a raised TSH, a condition known as subclinical hypothyroidism. Treatment with levothyroxine to lower TSH also improves conception in this condition.

A study published online in The Journal of Clinical Endocrinology & Metabolism on 19 December 2017 suggested that a slightly low thyroid hormone production that has responded to an increase in TSH that has risen to below the upper limit of non-pregnant normal may lie behind some cases of unexplained infertility.

The researchers analysed data from 239 female patients aged between 18 and 39 who had been diagnosed with infertility at a large academic health care system in Boston, USA between 2000 and 2012. They looked at TSH concentrations taken as part of the fertility evaluation from 187 women with unexplained infertility and 52 women whose male partners had inadequate sperm counts. Only women with regular menstrual cycles, a normal fertility evaluation and no history of thyroid problems were included.

Analysis showed that women with unexplained infertility had significantly higher normal TSH resultsthan women with male factor infertility. Twice as many women with unexplained infertility had TSH concentrations in the upper part of the normal reference range (26.9%) as women in the control group (13.5%).

The study’s senior author, Dr Pouneh K. Fazeli of Massachusetts General Hospital and Harvard Medical School, Boston, USA said “Since we now know from our study that there is an association between TSH levels at the high end of the normal range and unexplained infertility, it is possible that a high-normal TSH level may negatively impact women who are trying to get pregnant. This could open up new avenues for possible treatments. The next step will be to see if lowering TSH levels will help this group conceive.”