In this case-controlled, observational study, 24 subjects will participate in a 12-hour blood collection with a single blood draw at +24 hours. 12 men will have low testosterone levels (hypogonadism) and 12 men will have normal testosterone levels. Blood samples will be drawn every 2 hours and later analyzed for total testosterone and calculated free testosterone. The primary objective of this study is to assess the degree of diurnal variation in men with testosterone deficiency.

Inclusion Criteria:

Men with testosterone deficiency:

• Ability to read, write, and understand English
• Male sex at birth
• Age greater than or equal to 18
• Age less than 46
• Diagnosed with testosterone deficiency prior to visit 1
• Screening testosterone concentration of less than 350 ng/dL
• Willing and able to comply with the study protocol
• Willing to provide informed consent for this study
• No previous exposure to exogenous T unless off therapy for at least 4 weeks

Men without testosterone deficiency:

• Ability to read, write, and understand English
• Male sex at birth
• Age greater than or equal to 18
• Age less than 46
• Never diagnosed with testosterone deficiency
• Screening testosterone concentration of greater than 350 ng/dL
• Willing and able to comply with the study protocol
• Willing to provide informed consent for this study
• No previous exposure to exogenous T

Exclusion Criteria:

• Previous exposure to exogenous T, clomiphene citrate, or other Selective Estrogen Receptor Modulators, unless off therapy for at least 4 weeks
• Serious psychiatric disease or uncontrolled medical illness, as suspected from the history or clinical examination
• Used any sex hormones or steroidal anabolic drug supplements within 28 days before screening testosterone collection
• Incapable of giving informed consent or complying with the protocol

24 male subjects (12 with TD and 12 without TD)