Mild underactive thyroid (hypothyroidism): who should be treated?
Normal thyroid blood tests
- TSH: 0.5 – 5.0 mU/L. This is the most important test
- Free thyroxine (T4): 8.0 – 18.0 pmol/L.
How is an underactive thyroid gland diagnosed?
A high TSH and low thyroxine (T4) indicates an underactive thyroid gland (hypothyroidism) – and requires treatment with thyroxine (usually in the form of levothyroxine).
But. What about a mildly underactive thyroid gland?
Mildly underactive thyroid gland (hypothyroidism) is also called subclinical hypothyroidism.
This means it does not meet the whole definition of (definite) hypothyroidism. You may have no symptoms, and your thyroxine (T4) level is normal – meaning your body is making enough thyroid hormone.
Should a mildly underactive thyroid be treated?
There are potential downsides to treating subclinical hypothyroidism. There is the risk of overtreatment, which might cause symptoms, such as feeling jittery and insomnia. Also, long-term overtreatment can lead to loss of bone density.
If your TSH level is elevated to between 5 and 10 mU/L and your T4 is still in the normal range, you and your doctor will consider starting thyroid medication or repeating the TSH later on – especially if you have no symptoms.
If you have symptoms of hypothyroidism, or you have a positive test for anti-thyroid antibodies, you may want to start thyroid tablets right away. If you aren’t treated, your doctor should continue to monitor your thyroid function with blood tests every six to 12 months.
But. If your TSH level is higher than 10 mU/L, you should start treatment, because you will very likely develop symptoms of an underactive thyroid, even if you do not have them now.
