What is normal, really?(LETTERS)(Letter to the editor)

Dr. Rhoda H. Cobin's editorial ("What Is Normal Thyroid Function, Really?" Guest Editorial, Aug. 15, 2008, p. 9) prompts me to write. I have diagnosed and treated hundreds of patients, particularly in pregnancy when they demonstrate classic signs of hypothyroid disease or medical problems and have lower than expected thyroxine levels. I offer the following observations:

* Little attention is paid to the functional differences in thyroid assays from different companies, especially in distinguishing abnormal low values. Thyroid assays have not been standardized for pregnancy and at the very least need to be adjusted by gestational age.

* Multiple studies have indicated that increased rates of preterm labor, diabetes, macrosomia, placental abruption, weight gain, fatigue, and other complications are associated with low thyroid states.

* In my experience, thyroid-stimulating hormone (TSH) does not correlate with these clinical effects, and is useful only when outside the mid-normal range or above. Studies which obtain only the pituitary TSH do not accurately reflect thy roid function at the cellular level, only pituitary response. TSH likely identifies immune thyroiditis/primary hypothyroidism. It is clear that if one wishes to look one can find a much larger group of patients with "extrathyroidal" disease. I call this "subnormal hypothyroidism" (symptomatic, low free [T.sub.4], normal TSH).

* Free[T.sub.4] ([FT.sub.4]) seems to correlate with this clinical response. Although some re searchers advocate using the total [T.sub.4] in pregnancy for ...