Recent research has shown that there is a diurnal (daily) variation in TSH production with a peak during the night, declining to half of this during the afternoon. A further result of recent work has suggested that the tests should be undertaken in the morning and that the previous upper limit of normality was too high. Authorities in USA now suggest that 2.7mU/L should be the cut-off point and treatment should be offered above this.
In this country recent suggestions, sadly approved by the British Thyroid Foundation and certain senior endocrinologists, have placed the cut-off point now at 10mU/L, well above previous levels of 4 or 5mU/L, which were far too high anyway.
The thinking behind these proposed guidelines is beyond rational explanation; and the amount of illness and suffering which will result is heartbreaking. A problem with interpreting TSH results is that, firstly, it assumes that the hypothalamus and pituitary are working normally, and secondly, that the thyroid status is normal and is not being influenced by medication or other illness. It is further pointed out that the test must have a sensitivity of below 0.02 mU/L or false negatives may occur. Many tests are not as sensitive as this. An abnormally low result is usually the result of higher than normal levels of T4 (due to hyperthyroidism or over-dosage), and if associated with thyroid peroxidase antibodies Hashimoto’s disease is likely to be present.
The phenomenon of thyroid hormone resistance occurs at the receptor sites and is usually genetic. The patient may have persistently higher than normal levels of both T3 and T4, and a high TSH, though still not appearing to be hyperthyroid. These patients are sometimes wrongly diagnosed as hyperthyroid on the evidence of the blood tests alone. |
