In the case of low TSH just below the lower end of the normal range and T4 also just below the lower end of the normal range and T3 in range, but low, would you advise taking thyroid medication? No thyroid antibodies are present. Secondly, what could be the reasons for the low TSH?

Chris Kresser:  We’ll cover this in a little more detail when we talk about thyroid and blood chem, but when you see low TSH and also low T4 and T3, that can often be hypopituitarism. What happens is the pituitary output of TSH is lower than it should be, and that causes also a lower production of T4 and T3. So in that case, the TSH is not low because of negative feedback. It’s low because the pituitary output of TSH is low.

Another possibility is that the patient is taking thyroid medication with T3 in it and that suppresses the output of TSH and T4. You’ll often see low T4 and low or low normal TSH when the patient is already taking a T4/T3 combo. I’m assuming that’s not the case because the question was, should the patient take thyroid medication in the first place, but it’s worth pointing out. You should always ask if a patient is taking thyroid glandulars, desiccated thyroid, or any kind of OTC thyroid support if you see this kind of pattern.