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The uptake of technetium-99m (99mTc) by the thyroid was measured 15 min after injection using a directional counting technique. The 'washout' of thyroidal 99mTc after intravenous perchlorate injection was quantitated so that separate measurement of the extrathyroidal activity of 99mTc was unnecessary.In normal subjects the thyroidal uptake was 1\m=.\28\ m=+-\ 0\m=.\61% of the administered dose (mean \ m=+-\ 1 s.d.); in primary myxoedema 0\m=.\26\ m=+-\ 0\m=.\28%, in simple goitre 2\m=.\18\ m=+-\ 2\m=.\10%, in thyrotoxicosis 11\m=.\0 \ m=+-\ 5\m=.\4%, in hypothyroid patients with Hashimoto's disease 4\m=.\50\ m=+-\ 3\m=.\70% and in three patients with Pendred's syndrome the mean uptake was 9\m=.\3%. The technique has been evaluated and its advantages as a simple alternative to quantitative scintiscanning are discussed.

The uptake of technetium as pertechnetate by the thyroid is currently accepted as a useful index of thyroid function (Alexander, Harden & Shimmins, 1969). Although Pitt-Rivers & Trotter (1953) have shown thyroidal radioiodide concentration in the colloid using autoradiography, the intrathyroidal site of technetium concentration has not been determined previously.In this paper, we report the results of autoradiographic studies with [99Tc]pertechnetate in rat thyroid using a method which prevents diffusion of soluble radionuclides until exposure to the autoradiographic emulsion is completed.Four Sprague-Dawley albino adult male rats were used initially. Two rats received aminotriazole (0\m=.\1%) in their drinking water for 3 weeks before the study; the other two rats were on a normal diet. [99Tc]pertechnetate (300 \g=m\Ci ) was administered i.p. to each animal and after 1 h one animal in each group received 10 mg sodium perchlorate by the same route. Ninety minutes after radionuclide administration and under ether anaesthesia, the thyroid and trachea were rapidly removed together, frozen in isopentane, cooled in liquid nitrogen to about -170°C , and were sectioned (8 µ ) on a cryostat in the dark-room without thawing. The frozen sections were then placed directly on to slides previously covered with photographic emulsion (Rogers & Brown-Grant, 1971) and were exposed at -30°C for 5 days in a light-tight box. The emulsion was then developed in D 19B (Kodak Ltd) and fixed. Finally the pre¬ parations were stained by haematoxylin and eosin for examination under light microscopy.The procedure was repeated in one other rat given aminotriazole, though the sections were allowed to thaw before preparing the autoradiographs.The follicular arrangement of the thyroid can be recognized on histological examina¬ tion of both unstimulated (Plate, fig. 1) and stimulated glands (Plate, fig. 2). Auto¬ radiographs from those animals not given per chlorate disclosed that the major site of ["Tcjpertechnetate concentration was the colloid. There was little radioactivity over the follicular cells and interstitium. In the animals given perchlorate, no

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