What is TSH? 

If you have been diagnosed with Hashimoto’s or an underactive thyroid, then it is likely that the first marker mentioned to you by your doctor was something called TSH, the standard screening test for thyroid function. TSH stands for ‘Thyroid Stimulating Hormone’, and it is not produced in the thyroid but in the pituitary gland. It is essentially a summary report of how well the thyroid gland is working as assessed by the pituitary gland, almost like a boss’s report of you as an employee. Let us explain further, as unfortunately, the physiology of the thyroid and thyroid axis is a topic that can be confusing, even for those with a medical background! 

Pituitary and Thyroid Gland Link 

The pituitary gland is located within the brain, and is often called the “master gland”, as it is basically the “boss” of the thyroid. The pituitary produces TSH as a messenger molecule that tells the thyroid gland to make enough thyroid hormone. TSH sets off a chain reaction, which involves the thyroid cells getting the ‘green light’ to begin to build thyroid hormone. In addition, the pituitary gathers information from the thyroid as well and can release more or less TSH depending on the situation. This is known as a ‘negative feedback loop’. For example, if too little thyroid hormone is being produced, the pituitary gland can sense this and will therefore release more TSH. An elevated TSH translates into an underactive thyroid. If the pathology lies in the thyroid, then the relationship between TSH and thyroid hormones will always be inverse: when TSH is high, thyroid hormones will be low and vice versa.

Limitations of TSH 

TSH measures a composite of how well the thyroid has been working over a period of time. It will give an overall reading or “bird’s eye view” rather than day to day fluctuations of TSH. While it is a useful marker, it does not provide the whole picture of the thyroid. Also, because the TSH rises and falls only when there are fluctuations in thyroid hormone levels, it can often take some time before the pituitary senses the changes and starts to release more or less of TSH. Therefore, it is recommended that if one has symptoms of an underactive thyroid that there be a more complete blood panel done, including free T3 and free T4. In general, a ‘normal’ TSH count is between 0.4-4.5 uIU/ml, but this can vary according to the lab. It has been criticized that this lab range is too wide for most patients- ideally the optimal range is between 1.0-2.0 uIU/ml. If your TSH is interpreted as normal by your doctor, it is worth asking for the exact number to see if it is within the optimal range. Many patients who have a TSH over 2.5 will start to have symptoms of an underactive thyroid, despite being told that their thyroid function is normal.