The concept of subclinical hypothyroidism is based on the log-linear feedback loop between thyroxine and thyroid stimulating hormone: for one unit change in thyroxine there is a 10 unit change in thyroid stimulating hormone.
The prediction of disease by the measurement of an intermediate marker is now well established in the absence of clinical symptoms for example, calcium concentration, cholesterol concentration, and blood pressure.
It has been shown that changes in endothelial function and cholesterol concentration are apparent in subclinical hypothyroidism and even in people whose thyroid stimulating hormone is greater than 2 mU/l. (3,4) This provides a physiological basis for the Rotterdam study, which clearly shows that subclinical hypothyroidism is associated with an increased risk of ischaemic heart disease.(5)
A value outside the reference interval is not simply a minor variation but is important both in terms of predicting future hypothyroidism and in causing biological effects. (3,4)
Alun Price, chief medical laboratory scientific officer, clinical chemistry. A P Weetman, honorary consultant physician. Northern General Hospital, Sheffield, UK
If you have not read one of the best articles on this site, please review the hypothyroid article that I updated earlier this summer.
The most common traditional way to diagnose hypothyroidism is with a TSH that is elevated beyond the "normal" reference range. For most labs, the upper limit of the normal range is about 4.0 or even higher. This 'threshold' is thought to reflect the pituitary's sensing of inadequate thyroid hormone levels in the blood which would be consistent with hypothyroidism. There is no question that this will diagnose hypothyroidism, but it is far too insensitive a measure, and the vast majority of patients who have hypothyroidism will be missed.
Also be sure to read the article from the May 21 newsletter that discusses the original BMJ study (reference #1 below). CLICK HERE to see the full text of the study.
Symptoms of Low Thyroid
The most common is fatigue.
Skin can become dry, cold, rough and scaly.
Hair becomes coarse, brittle and grows slowly or may fall out excessively.
Sensitivity to cold with feelings of being chilly in rooms of normal temperature.
Difficult for a person to sweat and their perspiration may be decreased or even absent even during heavy exercise and hot weather.
Constipation that is resistant to magnesium supplementation and other mild laxatives is also another common symptom.
Difficulty in losing weight despite rigid adherence to a low grain diet seems to be a common finding especially in women.
Depression and muscle weakness are other common symptoms.
References
1. O'Reilly DStJ. Thyroid function tests - time for a reassessment. BMJ 2000; 320: 1332-1334. (13 May) 2. Weetman AP. Hypothyroidism: screening and subclinical disease. BMJ 1997; 314: 1175-1178. 3. Hak EA, Pols HAP, Visser T J, Drexhage HA, Hofman W, Jacqueline CM. Subclinical hypothyroidism is an independent risk factor for athe.rosclerosis and myocardial infarction in elderly women: the Rotterdam study. Ann Intern Med 2000; 132: 270-278. 4. Lekakis J, Papamichael C, Alevizaki M, Piperingos G, Marafelia P, Mantzos J, et al. Flow-mediated, endothelium-dependent vasodilatation is impaired in subjects with hypothyroidism, borderline hypothyroidsm and high-normal serum thyrotropin (THS) values. Thyroid 1997;7(3). 5. Michalopoulou G, Alevizaki M, Piperingos G, Mitsibounas D, Mantzos E, Adamopoulos P, et al. High serum cholesterol levels in persons with 'high-normal' TSH levels: should one extend the definition of subclinical hypothyroidism? Eur J Endocrinol 1998; 138: 141-145.
1. O'Reilly DStJ. Thyroid function tests - time for a reassessment. BMJ 2000; 320: 1332-1334. (13 May)
2. Weetman AP. Hypothyroidism: screening and subclinical disease. BMJ 1997; 314: 1175-1178.
3. Hak EA, Pols HAP, Visser T J, Drexhage HA, Hofman W, Jacqueline CM. Subclinical hypothyroidism is an independent risk factor for athe.rosclerosis and myocardial infarction in elderly women: the Rotterdam study. Ann Intern Med 2000; 132: 270-278.
4. Lekakis J, Papamichael C, Alevizaki M, Piperingos G, Marafelia P, Mantzos J, et al. Flow-mediated, endothelium-dependent vasodilatation is impaired in subjects with hypothyroidism, borderline hypothyroidsm and high-normal serum thyrotropin (THS) values. Thyroid 1997;7(3).
5. Michalopoulou G, Alevizaki M, Piperingos G, Mitsibounas D, Mantzos E, Adamopoulos P, et al. High serum cholesterol levels in persons with 'high-normal' TSH levels: should one extend the definition of subclinical hypothyroidism? Eur J Endocrinol 1998; 138: 141-145.
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