TSH is the most common test used to monitor hypothyroid patients who are treated with thyroid hormone replacement therapy. As thyroid hormone is increased (higher doses), the TSH level drops and rises when thyroid hormone is decreased (lower doses).

The question of whether a hypothyroid patient should take their thyroid hormone medication dose before a blood draw to monitor their replacement therapy is a common one. Patients wonder if taking the hormone dose will affect the lab results if it is taken only an hour or two before the blood is drawn. In most cases, I feel any slight change this would cause in blood lab results would not be significant or result in a change in the patient’s treatment. There are those less common cases however, when patients have more sensitivity to thyroid hormone therapy and even slight dose adjustments make a significant difference in how they feel. In these cases, the timing of their thyroid dose, in correlation with their blood draw retests, could make a difference in their treatment as their dose is adjusted.

It is certain in my opinion that anything ingested, including thyroid medication, has a “peak time” when it spikes in the blood stream and is at its highest level. This has to happen because the patient’s body will begin to metabolize the hormone dose, with the blood stream acting as a carrier for taking the hormone (and converted hormone) to the tissues of the body. Just before this happens, there must be a peak in the blood level even though that increased level may only vary slightly from the stable level that remains throughout the day. With this spike being only slight, would it really matter in regard to how the patient’s dose is adjusted? In my opinion, I feel it possibly could make a difference in the dose level their treating doctor will prescribe them, if it is a dose-sensitive patient as described above.

Synthroid (T-4 only) for example, has dose levels that can be adjusted at only 12mcg increments and Armour Thyroid (combo T-4 and T-3) can be adjusted at only 30mg increments (or even smaller by cutting pills). If a doctor is monitoring their patient’s dose using TSH, even a small spike in their blood level could result in their doctor not increasing their dose even though their stable level throughout the day, is only a few tenths of a point below that peak level. This would be reflected in the lab result from taking their dose within a couple of hours of having the blood drawn. If the patient instead takes their thyroid dose after the blood draw, the need for that slight dose increase might be more obvious to the treating doctor.

A better example would be a patient whose doctor likes to keep their thyroid hormone dose, at a level that keeps their TSH at between 1.0 and 2.0 but the doctor does not like to see the level over 2.0 or the patient risks a return of mild hypothyroid symptoms. If just before a blood draw, the patient’s TSH level drops down to 1.7 because of the spike in the blood stream from taking the dose within two hours of the blood retest their doctor might keep this patient’s dose level the same. This despite the fact that if the blood was drawn before the dose was taken, the patient’s TSH would have increased to slightly over a 2.0 and would signal a need to the doctor to slightly increase the patient’s dose.

As I stated at the onset of this article, the timing of a patient’s dose in correlation with blood draw for retests, is not usually significant unless they are the type patient whose dose must be kept in a narrow margin-level. There is a statement made by the manufacturer of Armour thyroid that recommends that patients not take their dose on the day of a blood draw, to monitor their therapy, until after the blood draw.
Here is a quote:
“If you are on a thyroid supplement medication and need to take a blood test, on the day of your blood test, you should wait to take your medication until after your blood has been drawn to avoid any test interference.” (Found on the “Diagnosis” page under the heading “Blood Tests”.).
A link to this page is posted below in the “Related Links” area.

Whether you decide to take your thyroid hormone dose before or after your blood is drawn for retests, is a decision that should be made between you and your doctor but the main thing is to be consistent in whatever method you use. This will insure consistency in your lab results.