How can I know if I have an underactive thyroid?

Tests and Symptoms. Symptoms of low thyroid function are commonly fatigue, weight gain, heavy periods (females), dry skin, hair thinning and loss, depression and constipation. Tsh, free T3 (liothyronine) and free T4 (if TSH is borderline) as well as some other older tests like morning baseline temperature and tendon reflex testing can all detect low thyroid function.