Thyroid Hormone, conception and miscarriage: here’s what the studies show regarding borderline low thyroid levels

There are many hormones that are involved in establishing and maintaining your health so that you can conceive and carry a pregnancy. One of the most important is the hormone called thyroxine which is produced by your thyroid gland. The role of this hormone is to regulate your metabolism much like the computer that regulates the idle speed in your car. If your thyroid hormone production is too low; you may feel sluggish like a car that is trying to stall whereas if your levels are too high you may feel overheated and agitated like a car whose engine is revving inappropriately. Although these examples of the extreme shifts of thyroid hormones are fairly easy to diagnose and treat; there are many subtle imbalances that may still hamper your fertility. An example is the condition called “subclinical hypothyroidism.”

Subclinical hypothyroidism is defined as the state of having an elevated level of TSH (thyroidstimulating hormone) but a normal thyroxine level. In other words, the brain is sending a signal to the thyroid gland to boost production of thyroxine but the thyroid gland isn’t responding fully. This imbalance occurs in about 8% of the general population but it is much more common in women with infertility. If left untreated, several studies suggest that overt hypothyroidism can result; having an adverse effect upon both the mother and her developing baby.  New studies are now providing much needed information to settle this debate on when and if this subtle hormone imbalance should be treated.

Earlier this year, a study reviewed the impact that treating subclinical hypothyroidism had upon the outcome of IVF treatment. Since the IVF process allows us to assess egg quality, fertilization, embryo quality and the implantation rate following embryo transfer, it offers us the most precise means to assess the impact of this therapeutic intervention. In this study patients with subclinical hypothyroidism were randomly assigned to treatment with thyroid supplementation or placebo. Those that received the hormone boost had better embryo quality, higher implantation rate and a reduced chance at having a miscarriage–all positive findings for couples trying to have a child.

Another larger study was published last year which was designed to evaluate the pregnancy loss rate as well as the preterm delivery risk in pregnant women with subclinical hypothyroidism. This was a “non intervention study” which means that they were just observing the outcome of the pregnancies without deciding when or who to treat. They found that women with untreated subclinical hypothyroidism had twice the risk of first trimester loss providing strong evidence to support the therapeutic intervention with thyroid supplementation.

Finally, there have been studies suggesting that women with subclinical hypothyroidism may develop low thyroid hormone levels if they consume a diet that is high in products made from soybeans (defined as more than 30g of soy protein per day which includes 16 mg or more of phytoestrogens). That’s because these estrogen-like chemicals produced by soy can reduce the ability of your body to convert thyroid hormone into its active form. In one study that actually looked at the effect of this dietary change, they found that 6 out of 60 patients developed low thyroid hormone levels due to a shift to a high soy diet. So although this healthy change in food choices can increase the risk of hypothyroidism it is not an inevitable change.

My recommendation is that if you have subclinical hypothyroidism that you monitor your condition closely and consult with your doctor as to whether or not you may benefit from supplementation with thyroxine (thyroid hormone) during your fertility treatment and pregnancy.