What is TSH (thyroid stimulating hormone)?

TSH, short for thyroid stimulating hormone, is vital for regulating your metabolism. That’s the process of changing the food you eat into well-managed energy distribution. TSH also plays a significant role in reproduction and pregnancy health: there may be a higher risk of miscarriage and other pregnancy problems when levels of the hormone are off-the-scale. Women planning IVF treatment should be routinely tested for TSH. As should women struggling to conceive naturally.

So what is TSH, in a nutshell? Like its half-sisters, FSH and LH, two other hormones IVF patients have to monitor, TSH is produced in the pituitary gland. All three share certain qualities. But while FSH and LH have their principle place of business in a woman’s ovaries, TSH is everywhere. It kick starts the thyroid gland in the neck to send out two enzymes, T4 and T3, to every corner of the body. These manage the body’s energy with ruthless efficiency.

If TSH controls metabolism, how does it affect fertility? Here’s how. Women below the age of 50 are far more likely to develop a thyroid problem than men. Many in this age range wish to have children. A normal TSH level can help guard against infertility and conception problems (ovarian and implantation-related, mainly), miscarriage, premature birth, pre-eclampsia, low birth weight and mental problems in the baby. The newly-developing fetus totally relies on its mother’s thyroid function. Optimal supply is best.

So what’s a normal TSH level? It depends where you live and who you listen to. In the UK, a practising range appears to be 0.27 to 4.2 mIU/L. Upper levels of 4.5 and, surprisingly, even 5.0 can be accepted elsewhere. Higher-than-normal levels may point to hypothyroidism: an underactive thyroid. A very low reading could suggest hyperthyroidism, an overactive one.

But some clinicians have argued that the upper TSH level shouldn’t exceed 3.0. Many fertility specialists also say that pregnant women should have a reading below 2.5 in the first three months of gestation and 3.0 after that – although a recent US study on hypothyroidism suggested IVF outcomes were not affected by slightly elevated levels. Dietary adjustments and a better exercise regime may help.

A newer study into thyroid disfunction found that 2.3% of women with hyperthyrodism had fertility problems, compared to 1.5% of the general population. That’s almost double. The research team advised regular screening, since daily medication can address most thryoid problems.

As far as IVF patients are concerned, it makes financial and medical sense to have your TSH levels tested. A TSH test is also advisable if you’ve had miscarriages, been trying to get pregnant for more than six months and/or have irregular bleeds. Or if there are thyroid issues in other members of your family.

TSH is the hormone many people forget about. Women with fertility problems should take the TSH blood test (on day 1, 2 or 3 of their cycle) at the earliest opportunity. It could make all the difference.