Classification of thyroid disorders
Thyroid disorders are classified as:
- primary—caused by a disorder within the thyroid gland, affecting the production or secretion of thyroid hormones (triiodothyronine [T3] and thyroxine [T4])
- subclinical—serum T3 and T4 concentrations are within the normal range, but serum thyroid stimulating hormone (TSH) concentration is outside the normal range. Subclinical disease is usually asymptomatic
- overt—serum T3, T4 and TSH concentrations are outside the normal range. Overt disease is usually symptomatic
- secondary—caused by a disorder within the anterior pituitary gland, affecting the production or secretion of TSH, which in turn affects the production of thyroid hormones
- tertiary—caused by a disorder within the hypothalamus, affecting the production or secretion of thyroid releasing hormone, which in turn affects the production of TSH and thyroid hormones.
In a primary disorder, the negative feedback response of serum T4 and T3 concentrations on release of TSH (and thyroid releasing hormone) remains intact, so primary hypothyroidism results in an elevated serum TSH concentration, and primary thyrotoxicosis results in a suppressed serum TSH concentration.
Secondary and tertiary thyroid disorders are referred to as central thyroid disorders. In a central thyroid disorder, the negative feedback response is dysfunctional; in hypothyroidism secondary to hypopituitarism, serum TSH concentration remains low or inappropriately normal despite low serum T3 and T4 concentrations. Central hyperthyroidism is rare.