First of all, I will define the term euthyroid that is often mentioned in this presentation. Euthyroid is the normal function of the thyroid gland, and corresponds to normal level of thyroid hormones in the blood.
Hypothyroidism denotes deficient production of thyroid hormones by the thyroid gland. It occurs when the thyroid hormones T4 and T3 levels fall below physiologically required levels. Usually (in the conventional way I should say), TSH level is above the "normal". It can be:
Primary hypothyroidism (abnormality in the thyroid gland itself): the most common form. It includes Hashimoto's thyroiditis (an autoimmune disease with thyroid antibodies), total or subtotal thyroidectomy, radioiodine destruction for hyperthyroidism, infiltrative diseases (sarcoidosis, amyloidosis, hemochromatosis...), or side effects of drugs, such as amiodarone or lithium.
Secondary/central hypothyroidism (as a result of hypothalamic or pituitary disease): it occurs if the pituitary gland does not create enough thyroid-stimulating hormone (TSH) to induce the thyroid gland to produce enough thyroid hormones. Although not every case of secondary hypothyroidism has a clear-cut cause, it is usually caused by damage to the pituitary gland, as by a tumor, radiation, surgery, infection, or by severe head trauma. Secondary hypothyroidism accounts for less than 5% or 10% of hypothyroidism cases.
The term "subclinical hypothyroidism" is used to define that grade of primary hypothyroidism in which there is a mild elevation of TSH concentration in the presence of normal serum free T4 and T3 concentrations. It is most commonly an early stage of hypothyroidism, usually asymptomatic (with no symptom). Subclinical hypothyroidism may progress to clinical (overt) hypothyroidism in approximately 2-5% cases annually. The term "subclinical" may not be strictly correct, since some of these patients may have clinical symptoms of mild thyroid failure, but no better term has been proposed. Many debates concerning whether to treat subclinical hypothyroidism, and on what criteria; I will discuss addressing treatment, the issue is important.
With higher than normal TSH levels and low free T4 levels, and/or low free T3 levels, symptoms become more readily apparent in clinical (or overt) hypothyroidism.