Subclinical Hypothyroidism in Infertility

The thyroid is a very important hormone responsible for the metabolism of the whole body. Thyroid hormone receptors are present all the reproductive organs, including the ovary, therefore it plays an important role in normal reproduction. Thyroid disorders are seen in 2 -4% of the women of reproductive age. Women are 4 to5 times more affected by thyroid disorders than men. Both the underproduction i.e. hypothyroidism, as well as overproduction i.e hyperthyroidism, have profound effects on onset of puberty, menstrual regularity, ability to conceive as well as miscarriages.

Evaluation of thyroid function is done by a simple blood test which measures TSH, freeT3 levels and freeT4 levels. Hypothyroidism is diagnosed when TSH levels are high and free T4 levels are low. High free T4 and low TSH  levels is diagnosed as hyperthyroidism. When TSH is high, but free T4 levels are normal, it is known as subclinical hypothyroidism.  The cut-off levels to define hypo or hypethyroidism varies among the different population, but generally, TSH levels at 4.0 miu/ml is considered normal.

The causes of thyroid disorders are iodine deficiency, anemia,  post surgical removal of the thyroid gland, radiation treatment, certain medications like antidepressants and antipsychotics. Transiently thyroid levels may fluctuate in certain viral infections, febrile illness, and burns. Subclinical hypothyroidism is most commonly caused due to autoimmune thyroiditis also known as Hashimotos thyroiditis . It is a genetic condition where autoantibodies are present in one’s body which act against own thyroid gland.

The diagnosis of subclinical hypothyroidism is suggested by family history, presence of anti-TPO or anti-TG antibodies in blood and a typical ultrasound of thyroid gland.  10-20% of the infertility patients have thyroid antibodies and around 60% of the patients with thyroid antibodies have infertility, however, it is still under research whether these antibodies could cause infertility or not. There is a lot of research which suggests that subclinical hypothyroidism is more common in patients with polycystic ovaries, endometriosis and unexplained infertility.

It is imperative to do an evaluation of serum TSH for all women seeking infertility treatment as the rising estradiol levels during the treatment unmasks subcinical hypothyroidism and leads to overt hypothyroidism. In patients already taking thyroid supplementation, dosage increase is needed during the initial stages of preganacy. Therefore, levels  of thyroid hormone should be repeated 2 -4 weeks after the IVF treatment, when it is an IVF pregnancy

During IVF in patients who have thyroid autoimmunity( subclinical hypothyroidism), a lower number of oocytes may be retrieved. Lower fertilization rate or lower number of good quality of embryos is also associated with thyroid autoimmunity.  Research has shown that the risk of miscarriage is increased upto 3 times in patients with uncorrected hypothyroidism with positive antibodies. The reasons behind negative associations between thyroid antibody and the reproductive outcome is still under research.  It is postulated that autoantibodies may block the entry of sperm in the egg, and hence initial results have shown that  ICSI ( intracytoplasmic antibody) technique may improve the outcome.

Therefore, screening for all women seeking fertility should be done for thyroid function.  Simple treatment with levothyroxine( LT4) tablet in cases with overt hypothyroidism and subclinical hypothyroidism( with autoantibodies) should be taken to improve the reproductive outcome.