High TSH with normal T3 and T4 is subclinical hypothyroidism, referred to as subclinical hypothyroidism. Subclinical hypothyroidism is an early stage of hypothyroidism, often found during physical examinations. Patients may have no symptoms or may have only mild hypothyroid symptoms. Some subclinical hypothyroidism may develop into hypothyroidism, while others may return to normal on their own, but some subclinical hypothyroidism may persist. The dangers of subclinical hypothyroidism can mainly cause hypercholesterolemia, aggravate the development of atherosclerosis, and also affect the intellectual development of the fetus. For the treatment of subclinical hypothyroidism, levothyroxine sodium tablets can be given as replacement therapy when TSH is greater than 10mU/L. When the TSH is between 4-10 mU/L, no treatment can be given and TSH changes can be monitored regularly. Patients with subclinical hypothyroidism who are pregnant or have fertility requirements need to be treated aggressively.