Considering OPK ....yay or nay?

O
I'm considering buying a opk this month. We're in our fifth month TTC #2 keep in mind we have a 3yr old we conceived right away and this is new to us since we got lucky. Then again we were younger & healthier. I'm now dealing doth hyperthyroidism & have been taking synthroid for 2yrs. 
Anyways back to the topic, Please advice on brands I'm not sure what yo buy. Thanks!