SAN FRANCISCO -- The chemist who created some of the steroids at the heart of a wide-ranging drugs and sports scandal testified Tuesday that he supplied the substances to competitive cyclist Tammy Thomas, then helped her craft a bogus explanation to cover up her doping.

Patrick Arnold said it was clear to him that Thomas wanted steroids that could not be detected by sports-testing labs at the time, in 2002.

"I believe Ms. Thomas understood full well that this was undetectable and that was its intention," Arnold said.

Prosecutors say Thomas lied to a grand jury about what performance-enhancing substances she had obtained from Arnold, who created the designer steroid THG that she and home run king Barry Bonds are accused of taking, and whether she had ever taken anabolic steroids. She is charged with making false statements and obstruction of justice.

Tuesday was the first full day of testimony in the first trial arising from the federal steroids probe that has implicated dozens of elite athletes. The testimony ranged from arcane discussions of human hormone science to detailed and deeply personal descriptions of how Thomas's body apparently changed after she took the substances.

Dr. Margaret Wierman, an endocrinologist at the University of Colorado, testified that when she examined Thomas in 2000, she observed masculine body features.

"My recollection was that when I examined her, she had specific signs of evidence of a full beard" and other features, including male-pattern balding, diminished breasts and unusual patterns of hair on her chest and arms, Wierman said.

Wierman also said she observed Thomas' deep voice, and wondered whether that would reverse itself later. Outside the courtroom, when Thomas chatted briefly with family and friends, her voice was noticeably deep.

Thomas sat poised and still during the testimony Tuesday about her physical transformation.

The doctor testified that Thomas denied taking any steroids or testosterone in a first consultation, but acknowledged in a follow-up conversation that she was "perhaps" regularly taking a steroid.

Wierman said she had "great concern" that Thomas had put herself at risk for long-term damage to her reproductive system and heart.

Thomas' attorney, Ethan Balogh, sought to suggest that Wierman was so busy during this time period teaching, treating patients, giving speeches and reviewing medical literature that she could not have given adequate attention to Thomas.

"Would you agree that you were a pretty busy woman in the year 2000?" Balogh asked.

"Not any busier than I am every year," she replied. Pressed by Balogh, Wierman allowed, "I was a busy woman in the year 2000."

Arnold pleaded guilty in 2006 to a charge of conspiring to distribute steroids and served three months in federal prison. The one-time amateur bodybuilder said he had effectively invented the performance-enhancing drug "the clear," a once-undetectable substance tetrahydrogestrinone, or THG. It is the same substance that Bonds is accused of taking.

Looking tense on the stand, Arnold said he had supplied Thomas with THG at least once and as many as three times. He also said it was possible his then-girlfriend had supplied Thomas with norbolethone, an obscure steroid, without his knowledge.

Kelcey Dalton, his former girlfriend, testified that she never shipped any drugs to Thomas, saying it was Arnold who personally mailed both THG and norbolethone to the cyclist.

Both Arnold and Dalton said they used the steroids themselves to improve weightlifting performances.

Dalton said she discussed the pros and cons of THG with Thomas, and that the two women talked about how to mask side effects such as acne and facial hair. Thomas had the "characteristic steroid voice" of women taking the substances, Dalton testified.

In a telephone conversation with Thomas after drug testing picked up norbolethene, Arnold recalled, "We brainstormed ways she could get out of the situation that she was in. I suggested to Ms. Thomas that a cover story could be contrived that involved telling the doping agency people that she was on a morning-after pill."

The legal pill had an "extremely close chemical structure to norbolethone and might be a plausible argument for her positive," Arnold testified, adding that Thomas seemed "interested in pursuing that avenue."

Asked by prosecutors whether Thomas knew that norbolethone was supposed to be undetectable, he answered: "Yes. That is surely why she was taking it, in my mind."

And questioned about what he knew of the side effects of the drugs, Arnold said it was impossible for him to say because he was giving it to people he didn't know who were taking other steroids at the time as well.

"They did not have a pristine physiology," he said.