Subclinical hyperthyroidism

What it is

Subclinical hyperthyroidism is a low TSH level with normal free T4 and free T3 levels. On the thyroid profile it occupies the row in which TSH is low and T4 is normal, whereas a low TSH with a high T4 is primary hyperthyroidism and a low TSH with a low T4 is central or secondary hypothyroidism. A low TSH with a normal fT4 may still be T3-mediated, which is possible in exogenous hyperthyroidism from factitious or incidental ingestion.

Higher on the same profile, a high TSH with a high T4 is central, or secondary, hyperthyroidism: a TSH secreting tumour, or thyroid hormone resistance giving increased feedback to the pituitary. A high TSH with a normal T4 is subclinical hypothyroidism, and in a patient taking levothyroxine that picture indicates poor compliance, white coat effect or a suboptimal dose. The last row is a low or normal TSH with a low or normal T4, which is sick euthyroid syndrome, seen in non-thyroidal illness; the note on that row says to repeat the thyroid function tests in 6-8 weeks.