Defining Subclinical Thyroid Disorders
Aug
26
Written by:
8/26/2008
Subclinical thyroid disorders are conditions in which levels of the pituitary hormone TSH are abnormal and levels of thyroid, T4/FT4 and T3/FT3, are normal. Understanding the role of TSH (thyroid stimulating hormone or thyrotropin) in regulating thyroid hormone levels makes subclinical disorders easier to explain.
The Pituitary-Hypothalamic-Thyroid Axis
Normally, the pituitary gland secretes TSH in short bursts throughout the day so that we always have an ample supply. TSH orders thyroid cells to grow and it orders thyroid cells to produce and release thyroid hormone into the bloodstream. In doing so, TSH is said to regulate thyroid hormone levels.
The hypothalamus in the brain watches over all our hormone levels. When it sees that more TSH is needed, it produces a hormone called thyrotropin releasing hormone (TRH). TRH orders pituitary cells (thyrotrophs) to produce more TSH (thyrotropin).
Normally, TSH levels will vary slightly throughout the day, according to the body's needs, in an effort to keep thyroid hormone levels stable.
When Things Go Awry
When, for whatever reason, the thyroid gland begins to produce excess thyroid hormone and levels start to rise, the hypothalamus notices.
In response, it produces less TRH, which, in turn, causes the pituitary to secrete less TSH. This is done in an effort to prevent thyroid hormone levels from rising too high. Thus, TSH levels will fall and become abnormally low before thyroid hormone levels rise abnormally high. During this timeframe, patients are said to have subclinical hyperthyroidism. A low TSH level causes no symptoms but it serves as a marker of the thyroid gland’s status.
Likewise, when thyroid hormone levels begin to fall, the hypothalamus notices. It produces more TRH, which, in turn, causes the pituitary gland to produce more TSH. This tells thyroid cells to produce more thyroid hormone.
Because of this regulatory axis, before levels of thyroid hormone fall abnormally low, the TSH level will rise abnormally high. During this timeframe, patients are said to have subclinical hypothyroidism. A high TSH causes no effects but serves as an indicator that thyroid hormone levels, while within the normal range, may be falling too low for the body’s needs.