New cases of thyroid abnormalities continue to turn up in Marshall Islands natives who 20 years ago were exposed to fallout from a U.S. hydrogen bomb test exploded more than 100 miles from their home atolls.

According to Interfor Department officials, doctors in the last 18 months have confirmed seven new thyroid cases in natives who lived in Rongelap and Utirik atolls on March 1, 1954, when the largest U.S. nuclear weapon, a 15-megaton hydrogen bomb, was exploded on the coral reef of Bikini atoll in the South Pacific.

The unexpectedly heavy radioactive fallout from the explosion was carried by winds to Rongelap, 110 miles east of Bikini. A lesser amount of fallout drifted an additional 180 miles east to Utirik.

Counting the most recent cases, 33 of Rongelap's 82 inhabitants at the time of the fallout have developed thyroid problems.

Five of them have been ruled cancerous and doctors believe many of the others would have turned malignant had not the victim's thyroids been surgically removed after the first sign of nodules.

In addition, the youngest child on the island at the time of fallout died of leukemia in 1972.

Of 21 Rangelap children under 12 years of age at the time of the explosion, 19 have turned up with thyroid tumoas or problems, according to an interior Department official.

The health problem of the exposed people on Rongelap and Utirik is but one aspect of the continuing controversy over the long-term effects of radiation.

Last week, another aspect of the problem emerged with disclosure that Bikini Island - which, though unoccupied, also took fallout from the March 1, 1954, explosion - is too radioactive for people to live on.

Bikini has been declared safe by the Atomic Energy Commission in 1969 and some 100 Bikinians had returned eight years ago. They will have to be moved to another island.

The health of the Marshallese who were dosed with radioactivity in 1954 has been closely monitored by U.S. government doctors in the 24 years since the fallout exposure.

Rongelap, according to scientists, received a total radiation dose of about 175 rads. But since the natives lived on the fallout-blanketed island for more than two days after the test - eating and drinking contaminated food and water - the inhaled and swallowed radiation doses to bones and organs such as the thyroid were considered much higher.

Utirik, since it was much further from the site of the shot, received a much lower dose - about 15 rads, according to AEC estimates made more than 20 years ago.

Where 175 rads were expected to cause some short-term medical problems, neither that dose nor the much smaller one on Utirik was thought dangerous over the long term.

Beginning the 10 years after the exposure, however, thyroid nodules, both malignant and benign, began turning up among the exposed natives in both atolls.

Of 157 people on Utirik at the time of the fallout, eight have developed thyroid tumors and four of those found to be cancerous.

The Utirik cases have come later than those on Rongelap and doctors suspect the lower dose received may take a longer time showing an effect.

Information on the new cases was disclosed last month when Interior officials asked Congress for an additional $600,000 to compensate the fallout victims who turn up with tumors.

Last year Congress approved a $1,083,000 compensation bill to pay the Rongelap and Utirik Marshallese whose illness had appeared between 1963 and 1976.

In their presentation last month, the officials said two more cases had developed on Utirik since 1976.

Five more cases are suspected in natives who were on the islands in 1954 and who will be examined during a medical survey beginning this week. TR 5

In addition, the Interior officials project that 10 to 15 more cases may appear before 1980, when authorization to make compensatory payments comes up again for congressional review.