Raised serum TSH in hypothyroidism.

It is desirable to detect early hypothyroidism of the mildest degree even before conventional tests of thyroid function become abnormal. Serum TSH levels (normal: undetectable to 4 muU/ml) rise in patients with mild hypothyroidism long before serum T4 and T3 levels fall. In the patient described the serum TSH level was 310 muU/ml, while other tests of thyroid function gave normal results. After treatment with thyroxine, serum TSH returned to normal. It should now be accepted that patients with mild hypothyroidism have a raised serum TSH and that thyroid insufficiency can be confidently excluded if the serum TSH concentration is normal. It is thus important to assay serum TSH when suspicion of hypothyroidism is aroused.