Levothyroxine replacement therapy in central hypothyroidism: A practice report

Because of lack of well-documented laboratory criteria, we assessed the use fulness of measuring free thyroxine (FT4) levels for monitoring levothyroxi ne replacement therapy in patients with central hypothyroidism. This consis ted of a retrospective review from 1991-1997 of patient profiles extracted into a Macintosh 4th Dimension data base from the medical information syste m at a tertiary care biomedical research facility. Information was also ret rieved from medical records of 135 ambulatory patients treated by 42 endocr inology practitioners, and 52 ambulatory patients treated by 20 endocrinolo gy practitioners for central and primary hypothyroidism, respectively. Pati ent profiles were reviewed for most recent thyroid function test results an d levothyroxine dosing information. Of 112 (83%) patients with central hypo thyroidism who had FT4 levels within the laboratory's reference interval, o nly 2 had a dosage change. The FT4 concentration was concordant with physic ian-assessed thyroid status in 65 (82%) of 79 patients (95% CI 72-90, p<0.0 2) for whom clinical assessment was available in medical records. Thyrotrop in, total thyroxine, and triiodothyronine levels were not significantly ass ociated with clinical status (p>0.12) in patients with central hypothyroidi sm. Despite similar demographic and levothyroxine dosing profiles, patients with central hypothyroidism had significantly lower serum FT4 and thyrotro pin concentrations than those with primary hypothyroidism. The appropriaten ess of levothyroxine replacement therapy in most patients with central hypo thyroidism is reflected by a normal FT4 concentration measured with a valid assay. Whether midnormal or upper normal values are necessary for optimal therapy, and whether the therapeutic goal should be different in children t han in adults, require prospective studies with independent, objective asse ssment of thyroid status.