**Core Concept**
Sex hormone-binding globulin (SHBG) is a protein produced by the liver that binds to androgens and estrogens, regulating their availability and activity in the body. SHBG synthesis is influenced by factors such as sex hormones, thyroid hormones, and insulin.

**Why the Correct Answer is Right**
SHBG synthesis is decreased in conditions characterized by insulin resistance and hyperglycemia, such as type 2 diabetes mellitus. This decrease is due to the inhibitory effect of insulin on SHBG production in the liver. Additionally, SHBG levels are also decreased in conditions with high circulating levels of androgens, such as polycystic ovary syndrome (PCOS). In these conditions, the increased androgen levels may lead to decreased SHBG production.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incomplete and cannot be evaluated.
**Option B:** This option is not a condition associated with decreased SHBG synthesis.
**Option C:** This option is incorrect because SHBG synthesis is actually increased in hypothyroidism, not decreased.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that SHBG levels are often decreased in conditions with insulin resistance and hyperglycemia, such as type 2 diabetes mellitus. This is because insulin inhibits SHBG production in the liver.

**Correct Answer:** D.