An underactive thyroid gland can cause extreme tiredness, as well as weight gain, constipation, dislike of cold temperatures, dry skin and hair, and depression. It is true that both underactive and overactive thyroid gland or inadequately treated thyroid problems can have adverse effects upon a pregnancy. When your blood result comes back, you will know whether this is an issue for this pregnancy or not. If your thyroid function is normal you will be reassured. If there is a thyroid abnormality, then you will be referred to a specialist so that you can receive prompt treatment. The sooner thyroid function is back within the normal range, the safer for the pregnancy.