we are trying for a baby soon. due to other health issues we are seeing a fertility endocrinologist. He advised us for pregnancy and a healthy baby it is best to get the TSH under 2. If it is high is can cause miscarriage.
You can, but it might be harder to conceive, or cause harm/create problems for the fetus if you do conceive. Your best bet is to start on thyroid replacement drugs if you haven't yet, and get your thyroid down to better levels before trying to conceive. And also ask about additional thyroid tests.
A TSH of 12 is high, and usually indicates hypothyroidism. Hypothyroidism occurs when your body does not produce sufficient amounts of thyroid hormone. Symptoms include the following: fatigue, weight gain, memory problems, dry skin, constipation, feeling cold all the time, depression, joint pain, low libido, and elevated cholesterol. Have you experienced these or any other symptoms lately?
As thyroid hormone is vital to almost all bodily processes, you will benefit greatly from further testing to determine the actual levels of thyroid hormone available in your body. The tests to request from your doctor are: Free T3, Free T4; TPOAB and TgAb are a good idea too, to see if you have the autoimmune disease that often causes hypothyroidism.
To answer your original question, here is the response I wrote to someone who asked the same question as you a while back, about their wife who had a TSH of 6:
Being hypothyroid will likely make it harder to conceive, yes. But more importantly, once she does become pregnant, low thyroid hormones will definitely impact the developing fetus. The earliest stages of fetal development is the time when it is most critical for the mother to have sufficient thyroid hormone.
http://www.ncbi.nlm.nih.gov/pubmed/24015847
"Conclusion: Early TH [thyroid hormone] insufficiency from maternal hypothyroidism affects offspring hippocampal development and memory."