Hyperthyroidism (overactive thyroid) is an autoimmune condition, like diabetes, lupus, psoriasis, rheumatoid arthritis. It can occur following stressful situations such as childbirth, loss, and any other environmental factors. I do believe that there is evidence that hypothyroidism runs in families but whether it qualifies as 'robust' I'm not sure. The way is starts is subtle at first, strange symptoms that a GP can misdiagnose. Even with 'normal' blood test results, some people find they are symptomatic or have a goitre. Only when it goes so far off the normal range, are people given thyroxine. One poster earlier said it can happen during menopause and I agree with that. It can make you feel like you're going crazy.