To the Editor.—
In the euthyroid state, thyrotropin (thyroid-stimulating hormone [TSH]) and circulating thyroid hormone control each other's secretion in balanced fashion. In primary hypothyroidism, thyroid hormone secretion fails and TSH levels rise progressively. With the previously mentioned thyroid physiology in mind, we analyzed our large thyroid database to examine and validate the levels of TSH and circulating thyroid hormone at which primary hypothyroidism manifests itself.
Results of thyroid function studies were obtained in 429 male and 134 female nursing home residents aged 60 years and older. Residents with known thyroid disease, acute illness, or exposure to iodine or contrast media within the last 30 days or who received lithium were excluded. Fasting venous blood samples were drawn between 7 and 9 AM. The free thyroxine index (FTI) (normal value, 2.2 to 4.6) was derived by multiplying the thyroxine concentration by the fractional triiodothyronine uptake (Nuclear Medical Laboratories,