Overview
Clinical Meaning
Subclinical hypothyroidism is a biochemical diagnosis: serum TSH is above the laboratory reference limit while free T4 remains within range.
Subclinical hypothyroidism is a biochemical diagnosis: serum TSH is above the laboratory reference limit while free T4 remains within range. The thyroid is beginning to produce less hormone, but increased pituitary stimulation is compensating enough to keep circulating free T4 normal. TSH is therefore the earlier signal of reduced thyroid reserve. “Subclinical” does not mean that the patient has no symptoms. Fatigue, constipation, dry skin, cold intolerance, low mood, and cognitive complaints may occur, but each has many alternative explanations. Symptoms alone cannot establish the diagnosis or determine the levothyroxine dose. The useful mental model is a thermostat demanding more heat from a furnace that is losing capacity. The room temperature, represented by free T4, is still maintained; the thermostat signal, represented by TSH, has risen. A single abnormal signal does not prove that the furnace is failing, because both the signal and the assay vary.