Agriculture Secretary Richard E. Lyng isn't tipping his hand, but insiders think he's decided to name Thomas O. Kay as undersecretary for international and commodity affairs when Daniel G. Amstutz leaves the department next month. The job is a USDA hot seat, involving trade issues and administration of the farm support programs.

Amstutz has not indicated what he'll do after he leaves USDA, but the word on the grapevine is that he might be persuaded to become chairman of the Commodity Futures Trading Commission. His career was in the commodity business before he joined the administration.

Kay now heads the Foreign Agricultural Service.Farmers' 2 Cents' Worth . . .

House and Senate members who thought they'd seen everything have learned something new. Their mail brought copies of a Dear Colleague letter from Rep. Byron L. Dorgan (D-N.D.) -- pasted to boxes of cornflakes and puffed wheat. Dorgan's message noted that corn growers receive only 2 cents from the 12-ounce box of cornflakes that retails for $1.29; wheat farmers receive 2 cents for their contribution to a 6-ounce, $1.29 box of puffed wheat. Dorgan, you might have surmised, is sponsoring legislation aimed at boosting farmers' income.Double Duty at HHS . . .

Steven Grossman, deputy assistant secretary of health and human services for planning and evaluation, has been given a second hat. At least for now, he will also be deputy assistant secretary for population affairs, in charge of the Family Planning Program, replacing Jo Ann Gasper, who was fired for insubordination in a dispute over abortion.

Grossman -- a former health adviser to Sen. Orrin G. Hatch (R-Utah) -- may have two jobs, but he still gets only one salary. Long if Not Lean . . .

Adm. Carlisle A.H. Trost, well-known booster of physical fitness, has ordered officers to pose for a full-length photo at promotion time. Starting Oct. 1, the current head-and-shoulders mug won't do.

The chief of naval operations apparently is tired of seeing beer bellies. His message ruled out coats and ties that might flatter those out of shape. "The uniform for official photographs shall be summer khaki in order to provide maximum photographic clarity," he wrote.Healthy Advice . . .

And there's more to say about good health: President Reagan, noting that the average American walks 115,000 miles in a lifetime, has urged the nation to observe the week of Aug. 2 as National Podiatric Medicine Week.

Meanwhile, Surgeon General C. Everett Koop, at Fort Leonard Wood, Mo., for a recent speech on AIDS, told 4,000 soldiers to use only American-made condoms. Government regulations ensure their quality, aides said.

Koop also pointed out that abstinence and monogamy are the best ways to avoid the fatal disease. Based on staff reports and news services