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TSH is a hormone released by the pituitary gland in the brain. TSH stimulates the thyroid gland to create the thyroid hormones, triiodothyronine (T3) and thyroxine (T4). This sets up a feedback loop: If T3 and T4 are high, TSH is typically low. If T3 and T4 are low, TSH is typically high. This means that having a high TSH is indicative of an underactive thyroid (hypothyroidism), and a low TSH is indicative of an overactive thyroid (hyperthyroidism) or a pituitary problem.
In places where there is adequate dietary iodine available, hypothyroidism is most commonly caused by an autoimmune disease. The symptoms include fatigue, cold intolerance, constipation, dry skin, and weight gain. Hypothyroidism can be treated with medication. Hyperthyroidism is also most commonly caused by an autoimmune disease. It can also be caused by overactive thyroid nodules. The symptoms include agitation, heat intolerance, increased bowel movements, sweating, weight loss, and a racing heartbeat. Hyperthyroidism can be treated with medication, radioactive iodine, and surgery.
What we mean by normal
In medicine, using the term “normal” can sometimes be off-putting. Saying something is “normal” implies that everything else is “abnormal.” Additionally, saying something is “normal” may not be accurate, since something that is “normal” for you may not be “normal” for somebody else. Therefore, instead of saying certain values are “normal,” alternative terminology may be to say that these values are “healthy” or “within the reference range.”
Additionally, some values have well-defined cutoffs, while others do not. For example, when looking at hemoglobin A1c levels, a value of 6.5 or greater is always diagnostic of diabetes. On the other hand, when looking at testosterone levels, some use cutoffs of 270–1,070 ng/dL while others use cutoffs of 300–1,000 ng/dL.
The information above represents values that are commonly used as cutoffs. However, depending on the specific source you’re looking at or the laboratory you go to, their values may be a little different.
Dr. Tzvi Doron is Board Certified in Family Medicine by the American Board of Family Medicine and is Ro's Chief Clinical Officer.