Early adaptation of thyrotropin and thyroglobulin secretion to experimentally decreased iodine supply in man

Five healthy male volunteers (aged 25 to 28 years) were studied both after 4 weeks of treatment with 200 μg iodine/d orally (PO) and following experimental iodine depletion by treatment with 3 × 300 mg perchlorate/d PO over a 4-week period, in an attempt to better define the early adaptive responses to an alteration in iodine supply in thyroid function. Intrathyroidal iodine, serum triiodothyronine (T 3), free T 3 (FT 3), thyroxine (T 4), free T 4 (FT 4), reverse T 3 (rT 3), thyroxine-binding globulin (TBG), thyroglobulin (Tg), and thyrotropin (TSH) levels (10-minute sampling over 24 hours) were measured at the end of iodine administration and at the end of perchlorate treatment. Thyroid volume was determined by sonography, and iodine content was determined by fluorescence scintigraphy. TSH pulses were analyzed by computer-assisted programs. Comparing both experimental situations, perchlorate treatment significantly reduced intrathyroidal iodine concentration (4.0 ± 1.3 to 3.0 ± 1.2 nmol/mL, P < .05), but thyroid volume and total serum T 4, T 3, FT 3, and TBG levels were not altered. Mean 24-hour serum TSH levels (1.8 ± 0.3 to 1.0 ± 0.3 mU/L, P < .001), amount of TSH secreted/pulse (0.5 ± 0.1 to 0.3 ± 0.1 mU/L, P < .001), and FT 4 levels (15.7 ± 1.7 to 14.3 ± 1.4 pmol/L, P < .005) were significantly diminished, whereas Tg levels (18.6 ± 10.0 to 35.1 ± 14.0 ng/mL, P < .01) were significantly increased. Thyroid-specific antibodies were normal and were not altered by treatment. These data suggest a higher sensitivity of the thyroid to TSH in the early adaptation to iodine depletion; thus, less TSH is sufficient to maintain normal thyroid function.