My bf is born without a thyroid side effects n does it course sexual problem s?

No. Thyroid controls metabolism but not sexual development or maturation. Pituitary controls both thyroid & testes but if he's only missing thyroid, then testes should be ok. However, if testosterone is low, need to figure out why. Low t might be due to testicular issue (coincident w/thyroid) or be due to pituitary problem (see above) or perhaps liver (via excess iron - hemochromatosis), etc. Go c doc.