Alexander C. Chester, MD

In 95% of cases low thyroid hormone levels in a person are the result of the failure of the gland itself rather than a failure of the gland to receive the appropriate signal (thyroid stimulating hormone) from the pituitary gland. To correct a low thyroid state thyroid hormone is given as T4 in a tablet form calibrating the dose by measuring the thyroid stimulating hormone (TSH) which exists in a reciprocal relationship with T4. In other words, TSH is elevated when the thyroid dose is too low and TSH is suppressed when the thyroid dose is too high.

Thyroid hormone is given as T4 which is a prohormone converted to the active hormone (T3) by the body. Over the years some authorities have claimed that giving thyroid supplementation as a combination of T4 and T3 is superior to administering T4 alone. Multiple studies have not demonstrated any advantage of the combination preparation. Additionally, producing a standardized dose of this preparation is more difficult and subject to variation.

There is about a six week lag between an adjustment in T4 and the final change in the TSH level. Therefore, one cannot assume that more thyroid medication is required after a dose increase because of a failure of TSH to normalize until six weeks have passed. Additionally, T4 has a seven day half-life which insures a constant blood level throughout the day even though the tablet is taken only once a day and makes changes in blood level slow to appear.

Although TSH is used as the marker for thyroid dose adjustment checking the actual level of free thyroid hormone in the blood is important when initiating therapy and occasionally over the years to monitor therapy.

As mentioned in a previous blog, there is some debate about what is the proper TSH target. Traditionally normal TSH is defined as about 0.5 to 5 mU/L. These normal values are based on wide population studies that, by necessity, include individuals with low thyroid hormone levels and, therefore, elevated TSH levels. Using a proper sample population that excludes all individuals with thyroid abnormalities produces a value of 2.5 mU/L as an upper limit for TSH. It may be possible for a person to feel not quite right with a TSH of 4.9, for example, and to feel better with a TSH of 1.9.

Even when an appropriate dose of thyroid is started other factors can interfere with delivering the correct dose to the blood stream. The amount of hormone delivered to the blood steam from a particular thyroid hormone preparation (bioavailability) is standardized by the pharmaceutical firm and supervised by the government. While it used to be thought that a particular name brand was more reliable this has not proven to be the case. However, different generic preparations can have different bioavailability. If there is some concern after a different generic has been started checking the TSH will determine if altered bioavailability is an issue.

Thyroid hormone may not be properly absorbed thereby confusing attempts to find the proper dose. Ideally, it should be taken on an empty stomach one hour before breakfast or in the evening two hours after the last meal. Taking it with food or even coffee can diminish absorption. Taking it with certain other medications or supplements such as calcium, iron or Prilosec can interfere with absorption.

Many medications interfere with T4 becoming active in the blood steam even if they are not taken simultaneously with the thyroid dose. Practically speaking, if a person is on certain medications that interact with thyroid medication the dose can be altered to correct the issue. Problems usually only arise when a new interfering medication is initiated and the TSH is not checked to make the necessary adjustments in dose.

Your thyroid dose may need adjustment upwards if you put on weight. Additionally, your thyroid may be producing some hormone that might decrease over years requiring an increase in the thyroid dose. In any case, it is important to check the TSH yearly and, of course, if symptoms develop to insure that the thyroid dose is proper.

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