Use of bromocriptine in a patient with generalized resistance to thyroid hormone

A 50-yr-old man with a large goiter had elevated serum levels of T4 (22 micrograms/dl), T3 (228 ng/dl), and free T4 (6.6 ng/dl), a high 24-h 131I-thyroidal uptake (69%), and an elevated TSH level (3.6 microU/ml), as well as hyperresponsiveness of TSH to TRH (peak TSH, 47 microU/ml). Normal serum alpha-subunit concentrations and sella turcica films mitigated against the presence of a pituitary tumor. A normal basal metabolic rate and minimally elevated concentration of testosterone-estradiol binding globulin were consistent with generalized resistance to thyroid hormone. Mild lactotroph resistance was also present. The patient was given bromocriptine for 16 months (2.5-10 mg daily). Basal serum TSH levels decreased (less than 0.3-2 microU/ml), as did the TSH response to TRH; serum T4 levels (14-17 micrograms/dl); and 24-h 131I-thyroidal uptake (28%) were reduced. Serum T3 levels, however, changed little. The thyroid gland decreased to normal size, and the basal metabolic rate and testosterone-estradiol binding globulin remained normal. Bromocriptine was stopped for 4 months. Serum TSH increased to 4.5 microU/ml; T4 and T3 increased to 27 micrograms/dl and 328 ng/dl; the patient became clinically mildly hyperthyroid. Thus, bromocriptine in this patient was useful in decreasing TSH secretion and decreasing goiter size, while maintaining clinical euthyroidism. Only a small amount of TSH was necessary to maintain iodine uptake and thyroid hormone synthesis and secretion in a responsive thyroid gland. We speculate that this patient may be secreting a highly bioactive form of TSH and/or have increased thyroid sensitivity to TSH.