Subclinical hypothyroidism

What it is

Subclinical hypothyroidism is read off the thyroid profile: TSH is raised while T4 stays normal. It is the likely diagnosis in an asymptomatic patient with a slightly elevated TSH despite normal free T3 and T4 levels, and it is defined by normal T3 and T4 levels with a mildly elevated TSH and no symptoms of hypothyroidism. It sits one step short of primary hypothyroidism, where the raised TSH comes with a low T4 and autoimmune thyroiditis is the usual cause. A low TSH with a normal T4 is the other subclinical state, subclinical hyperthyroidism. One note belongs here. In a patient already taking levothyroxine, the same raised TSH with a normal T4 indicates poor compliance, white coat effect or a suboptimal dose rather than a new diagnosis. The full grid it sits in reads like this. A high TSH with a low T4 is primary hypothyroidism, usually autoimmune thyroiditis; a high TSH with a normal T4 is subclinical hypothyroidism; a high TSH with a high T4 is central, that is secondary, hyperthyroidism, from a TSH secreting tumour or from thyroid hormone resistance with increased feedback to the pituitary. A low TSH with a high T4 is primary hyperthyroidism, such as Grave's disease, and it is also possible in exogenous hyperthyroidism from incidental ingestion, which is T3-mediated if fT4 is normal; a low TSH with a normal T4 is subclinical hyperthyroidism; a low TSH with a low T4 is central, secondary, hypothyroidism, from pituitary or hypothalamic pathology, the non-secretory lesions. A low or normal TSH with a low or normal T4 is sick euthyroid syndrome, seen in non-thyroidal illness.