What does a high or low TSH mean?
TSH is not a thyroid hormone. It is the order sent to the thyroid, which is why its number moves the opposite way to what you expect.
The thyroid is a small gland in the front of the neck, and its hormones control how the body uses energy. The pituitary gland in the brain tells the thyroid how hard to work, using a hormone called TSH.
Why does the number go the opposite way?
When thyroid hormone is low, the pituitary makes more TSH to push the thyroid to work. When thyroid hormone is high, the pituitary cuts TSH back. So:
- A high TSH usually points to an underactive thyroid.
- A low TSH usually points to an overactive thyroid.
In a few cases the problem is in the pituitary itself.
What are the symptoms?
Underactive: tiredness, sensitivity to cold, weight gain, constipation, dry skin, hair loss, low mood and heavy or irregular periods. These begin slowly and may go unnoticed for months or years.
Overactive: weight loss despite a good appetite, a fast heartbeat, nervousness, trouble sleeping, shaky hands, sweating and sensitivity to heat. Over the age of 60 the signs can differ, such as loss of appetite or withdrawing from people, and may be mistaken for depression.
What comes next?
TSH says the thyroid is working too much or too little, but not why. If your result is abnormal, your doctor usually asks for T4, T3 and sometimes thyroid antibodies. These antibodies are checked for the autoimmune conditions Hashimoto’s disease and Graves’ disease.
If you take thyroid medicine
An underactive thyroid is usually treated with hormone replacement tablets, often for life. Regular TSH tests show whether the dose is right, because the wrong dose causes side effects. Do not change your dose yourself, and tell your doctor about all your medicines.