**Core Concept**
Sex hormone-binding globulin (SHBG) is a protein that binds to sex hormones, such as testosterone and estradiol, in the bloodstream, regulating their bioavailability and activity. The levels of SHBG are influenced by various factors, including hormonal, metabolic, and nutritional changes.

**Why the Correct Answer is Right**
Insulin is known to decrease sex hormone-binding globulin (SHBG) levels. This effect is mediated through insulin's action on the liver, where it inhibits the production of SHBG. The decrease in SHBG levels leads to an increase in the bioavailability of testosterone and estradiol, which can contribute to various physiological and pathological processes. The mechanism of insulin's action on SHBG is not fully understood, but it is thought to be related to insulin's role in regulating the expression of genes involved in SHBG production.

**Why Each Wrong Option is Incorrect**
**Option A:** Oral contraceptive pills actually increase SHBG levels, not decrease them. This is because estrogen in oral contraceptives stimulates the production of SHBG in the liver.

**Option C:** Thyroxine hormone, or T4, has a complex relationship with SHBG levels. While some studies suggest that T4 can decrease SHBG, others have found no significant effect. Therefore, it is not a reliable option.

**Option D:** Pregnancy is associated with an increase in SHBG levels, not a decrease. This is thought to be due to the increased levels of estrogen and progesterone during pregnancy, which stimulate the production of SHBG in the liver.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to understand the complex interplay between insulin, SHBG, and sex hormones, as alterations in SHBG levels can have significant implications for reproductive health and metabolic function.

**✓ Correct Answer: B. Insulin**