642

form was decreased proportionally to the increase oftotal T4. This sequence of changes can be arguedbecause there is an equilibrium between a hormone andits binding proteins.’4 The FT4 index is, in essence,T4xT3 resin uptake. T3 resin uptake is, for practical

... purposes, taken to represent the fraction of unboundT4, although T3 tracer, is being used. There is no objec-tion to this procedure and it is permissible to use theFT4 index (as an estimate of FT4) for comparisonbetween patients, provided that the affinity character-istics of the binding proteins do not differ between thesubjects tested. If serum binding of T4 is increased butserum binding of T3 is not, FT4 index will give (falsely)raised values because the unbound fraction of T4, asestimated with T3 tracer, will not be found to be de-creased. Indeed we found that the affinity of T4 for thethree thyroid-hormone-binding proteins in the subjectspositive for the trait was higher than that in their unaf-fected relatives. Although the apparent Ka values of theaffected and unaffected members are of the same orderof magnitude as those reported by others ’20 the differ-ences between the family subgroups are highly signifi-cant. The finding that the affinity of T4 for all threebinding proteins is increased, not just for one, is remark-able and suggests either that a circulating substance in-terferes positively with the interaction between T4 andT.B.G., T.B.P.A., and albumin, or that there is a defi-

ciency of a natural occurring inhibitor.These possibilities, however, are speculative and

further investigations are necessary. Because T.B.G. andT.B.P.A. capacity and albumin concentration was normalin all the members of both families who had the abnor-mal thyroid hormone profile, the increased affinity of T4would have led to increased occupancy of binding sitesby T4 and consequently a decreased number of freebinding sites. Serum-T3 and serum-rT3 were normaland tracer T3 binding to serum was unaltered (as shownby the T3 resin uptake). If fewer sites for binding ofthese hormones were available, the affinity of these otheriodothyronines for the three serum proteins was, pre-sumably, proportionally increased relative to T4.The results of the turnover studies in the propositi

accorded with our in-vitro findings of increased serumbinding of T4 with a normal FT4 and normal serumbinding of T3 and rT3. The decreased VT4 and in-creased PT4 are fully explained by increased serumbinding of T4 and are of the order described in subjectswith idiopathic elevation of serum T.B.G.21 The normalturnover of T4 is consequently expected when FT4 isnormal.The frequency in the population of the "euthyroid

high total-T4, normal T3 syndrome" is unknown but,from a practical point of view, its existence should be

kept in mind in the diagnostic evaluation of thyroidfunction in patients.We thank Dr C. v. d. Peyl, Juliana Hospital, Terneuzen, and Dr

B. A. de Planque, Municipal Hospital, Dordrecht, for the opportunityto study their patients, and Miss B. Engelhard for secretarial assist-ance.

Requests for reprints should be addressed to G. H.

REFERENCES

1. Turner, J. G., Brownlie, B. E. W., Sadler, W. A. Lancet, 1975, i, 407.2. Kirkergaard, C. G., Siersbæk-Nielsen, K., Friis, Th., Rogowski, P. ibid. p.

868.

3. Hadden, D. R., McMaster, A., Bell, T. K., Weaver, J. A., Montgomery,D. A. ibid. p. 754.

4. Turner, J. G., Brownlie, B. E., Sadler, W. A., Jensen, C. A. ibid. p. 1292.5. Button, K. E., Quin, V., Ellis, S. M., Cayley, A. C. D., Miralles, J. M.,

Brown, B. L., Ekins, R. P. ibid. p. 141.6. Surks, M. I., Schadlow, A. R., Stock, J. M., Oppenheimer, J. H. J. clin.

Invest. 1973, 52, 805.7. Birkhäuser, M., Burer, Th., Busset, R., Burger, A. Lancet, 1977, ii, 56.8. Visser, T. J., van den Hout-Goemaat, N. L., Docter, R., Hennemann, G.

Neth. J. Med. 1975, 18, 111.9. Docter, R., Hennemann, G., Bernard, H. F. Israel. J. med. Sci. 1972, 8,

1870.10. Visser, T. J., Docter, R., Hennemann, G. J. Endocr. 1977, 73, 395.11. Digulio, W., Michalak, Z., Wemhold, P. A., Hamilton, J. R., Thomas, G. E.

J. lab. clin. Med. 1964, 64, 319.12. Van Welsum, M., Feltkamp, T. E. W., de Vries, M. J., Docter, R., van Zijl,

J., Hennemann, G. Br. med. J. 1974, iv, 755.13. Karlsson, F. A., Wibell, L., Wide, L. New Eng. J. Med. 1977, 296, 1146.14. Robbins, J., Rall, J. E. Physiol. Rev. 1960, 40, 415.15. Hennemann, G., Docter, R., Dolman, A. J. clin. Endocr. Metab. 1973, 33,

63.16. Hennemann, G., Smeulers, J., van der Does, I., Docter, R., Visser, T. J. Acta

endocr. Copenh. 1976, 82, 92.17. Oppenheimer, J. H., Schwartz, H. L., Surks, M. I. J. clin. Endocr. Metab

1975, 41, 319.18. Oppenheimer, J. H., Schwartz, H. L., Surks, M. I. ibid. p. 1172.19. De Groot, L. J., Stanbury, J. B. in The Thyroid and its Diseases; p. 213,

1975.20. De Groot, L. J., Stanbury, J. B. ibid. p. 64.21. Nicoloff, J. T., Low, J. C., Dussault, J. H., Fisher, D. A. J. clin. Invest. 1972,

51, 473.

Addendum

After we told doctors in our hospital about the syn-drome, we were presented with 3, probably 4, additionalsimilar patients, of whom 3 had been previously treatedwith antithyroid drugs for supposed hyperthyroidism.

If this syndrome turns out to be a common one, somethought should be given to abandoning the FT4 indexas a routine screening test for thyroid function andusing the measurement of the FT4 instead.

VISUAL DISABILITY AND HOME LIGHTING

T. R. CULLINANE. S. GOULD

J. H. SILVERD. IRVINE

Department of Environmental and Preventive MedicineMedical Research Council Toxicology Unit, St. Bartholomew’s

Hospital Medical College, London EC1M 7BE

Summary 13 men and 43 women (average age 76)attending a low-vision clinic with visual

acuity of 6/18 (Snellen) or less had acuity measurementsmade under standard (measured) hospital conditions,under normal home conditions, and under home condi-tions with augmented lighting. Median levels of ambientlighting in the home were 1/10 of those in hospital,while levels for reading were 1/7. Augmented lighting athome (a 60 watt bulb in a small adjustable lamp) im-proved visual acuity in 82% of subjects, restoring all but11% to the levels achieved in hospital or above. Im-provement was unrelated to disease. General levels of

lighting are often so poor in the homes of elderly peoplethat the number of people functioning as "blind" istwice what it need be. Simple improvements to lightingwould reduce the prevalence of "visual disability" (lessthan 6/18 Snellen) from 520/100 000 home-based adultsto about 300.