TSH – Understanding Thyroid Function Tests

Photo from endocrineweb.com

TSH is the abbreviation for Thyroid Stimulating Hormone.  Measurement of TSH level is the standard test used by physicians to both screen for and monitor treatment of hypothyroidism.  Understanding the TSH test results is quite simple once you know how the body regulates your thyroid function.  The process begins in the hypothalamus, the portion of the brain just above the pituitary gland.  The hypothalamus senses the level of thyroid hormone, and drives the process.  If you need more thyroid hormone, the hypothalamus secretes a hormone called Thyroid Releasing Hormone (TRH) into the microcirculation from the hypothalamus, down the stalk of the pituitary and to the pituitary gland.  The TRH signals the pituitary to secrete TSH into the bloodstream. The TSH goes through the bloodstream and stimulates the thyroid gland to put out thyroid hormone. So if your body needs more thyroid hormone as determined by the hypothalamus, the amount of TSH released by the pituitary increases dramatically.  In most symptomatic hypothyroid patients the TSH level is markedly elevated. A normal TSH level is approximately 0.4 – 4.5.  In hypothyroidism typically the TSH level will be much higher, with levels of  15-300 quite common.  We have more trouble deciding the significance of minor elevations of TSH, in the <10 range.  In patients on thyroid replacement therapy for hypothyroidism most physicians aim for a TSH level in the low normal range, from 0.4-2.0.  If patients feel well with upper normal TSH levels these are fine too.  (range varies from lab to lab, by gender, in pregnancy, etc.) Most experts recommend monitoring a TSH level about annually in patients on thyroid replacement therapy as the dose required can vary from time to time in any individual.

So it’s really simple:  An elevated TSH indicates an underactive thyroid or under-treated hypothyroid condition.  A low, or suppressed, TSH indicates an overactive thyroid, or an over-treated hypothyroid state.  Sometimes, especially with a slightly suppressed TSH it is helpful to measure the levels of thyroid hormone in the bloodstream.  In past years this was difficult, due to the large range of normal for total T4 and total T3 in different individuals.  This huge range is in part because most of the circulating thyroid hormone is protein bound, and not available at any given moment to bind to the thyroid hormone receptors.  Prior to the development and refinement of the TSH assay for routine use this was a big problem.

Fortunately in addition to the development of the highly sensitive TSH assays, we now have affordable measures of Free T4 and Free T3 levels. T4, or levothyroxine, is the thyroid hormone stored and released by the thyroid gland in largest quantities.  T3, or triiodothyronine, is the more physiologically active thyroid hormone, and is a metabolite of T4.  Both T3 and T4 circulate in the bloodstream both as protein bound and free or non-protein bound hormones.  The free, or unbound form is the form that functions to control metabolic rate.  In years past, before we had lab measurements of Free T4 and Free T3 we estimate the free amount by what was called the Free Thyroxine Index, a number calculated by multiplying the total T4 by the thyroid binding capacity.   This was notoriously inaccurate and really plays no role in modern thyroid management.  I still see non-endocrinologist, non-primary care physicians order a FTI (also called T7) level at times.  Now in the circumstance when we need to know the level of circulating thyroid hormone levels we order a Free T4, and sometimes a Free T3 level and avoid the confusion of protein bound thyroid.

So now you know:

High TSH à Low thyroid hormone status.

Low TSH à High thyroid hormone status.