rs121908866 — TSHR W546X

TSHR W546X — A Nonsense Mutation That Silences the Thyroid's Master Switch

The thyroid gland cannot make or release hormones on its own; it requires a continuous signal from the pituitary gland in the form of thyroid-stimulating hormone (TSH). TSH works by binding to its dedicated receptor — the TSH receptor (TSHR)¹¹ TSH receptor (TSHR)
a G protein-coupled receptor embedded in the membrane of thyroid follicular cells; its activation triggers a cascade that produces thyroxine (T4) and triiodothyronine (T3) — and triggering the intracellular machinery that produces thyroid hormones. When TSHR is non-functional, the pituitary sends ever-louder signals (elevated TSH) to a thyroid that cannot respond. The rs121908866 W546X variant replaces tryptophan at codon 546 with a premature stop signal, truncating the receptor protein and eliminating functional TSH signaling.

The Mechanism

The TSHR protein spans the cell membrane seven times — its extracellular loops bind TSH, and its transmembrane helices and intracellular loops relay the signal to adenylyl cyclase, triggering cAMP production. Tryptophan 546 sits in the fourth transmembrane domain, a critical structural region. The W546X stop-gain²² W546X stop-gain
creates the nonsense sequence TGG→TAG, introducing a premature termination codon that produces a severely truncated protein lacking the fifth, sixth, and seventh transmembrane helices, the third intracellular loop, and the entire C-terminal tail. Functional studies confirm that this truncated receptor cannot reach the cell surface and has negligible TSH binding activity. The result is complete TSH resistance: even massively elevated pituitary TSH output cannot drive thyroid hormone synthesis.

Because W546X is a null allele, the clinical phenotype follows a dose-response pattern determined by how many functional TSHR copies remain. Two W546X alleles (or one W546X combined with another inactivating mutation) leave the thyroid entirely unable to respond to TSH, producing severe congenital hypothyroidism. One W546X allele, with a working copy still present, allows partial receptor signaling — but the halved receptor density means the thyroid must work harder under higher TSH drive to maintain normal hormone output.

The Evidence

The clearest picture of W546X came from a Welsh neonatal screening study by Jordan et al. 2003³³ Welsh neonatal screening study by Jordan et al. 2003
W546X mutation of the thyrotropin receptor gene: potential major contributor to thyroid dysfunction in a Caucasian population. J Clin Endocrinol Metab, 88:1008–12. Two siblings detected through newborn screening were homozygous W546X and showed complete inability to respond to TSH. Screening of 368 euthyroid Welsh individuals identified two heterozygous carriers (G:A), giving an estimated heterozygous carrier frequency of approximately 1 in 180 in that Welsh population — substantially higher than the global gnomAD estimate of 0.03%, suggesting the mutation may be enriched in some European subpopulations.

In a compound heterozygosity study by Park et al. 2004⁴⁴ compound heterozygosity study by Park et al. 2004
Congenital hypothyroidism and apparent athyreosis with compound heterozygosity for inactivating TSHR mutations. Clin Endocrinol, 60:220–7, two siblings carried one W546X allele from their mother and one A553T allele from their father. The affected children had severe congenital hypothyroidism mimicking complete thyroid agenesis; the W546X-heterozygous mother had compensated hypothyroidism with thyroid hypoplasia. This family demonstrates the spectrum of TSHR inactivation: the severity scales with the degree of combined receptor loss.

Tenenbaum-Rakover et al. 2015⁵⁵ Tenenbaum-Rakover et al. 2015
Long-term outcome of loss-of-function mutations in thyrotropin receptor gene. Thyroid, 25:292–9 followed 94 subjects across 11 years and found that heterozygous TSHR mutation carriers showed only mild, stable TSH elevation without progression to overt hypothyroidism in most cases. Homozygous patients, however, showed declining free T4 levels over time, often requiring levothyroxine. This long-term natural history data supports periodic monitoring for heterozygous carriers rather than immediate treatment.

The reproductive significance stems from the well-established connection between maternal thyroid function and fertility, implantation, and early pregnancy. Subclinical hypothyroidism — defined as elevated TSH with normal free T4 — is associated with reduced conception rates, increased miscarriage risk, and impaired IVF outcomes in some (though not all) studies. Carriers of TSHR inactivating mutations who develop subclinical hypothyroidism are therefore a population in whom pre-conception TSH screening has particular clinical rationale.

Practical Actions

For heterozygous carriers, the key question is whether mild TSH elevation (common but not universal in this group) warrants treatment, particularly around pregnancy. Current endocrine guidelines recommend TSH < 2.5 mIU/L before and during the first trimester of pregnancy. A TSH above this threshold — whether from TSHR haploinsufficiency or other causes — may merit low-dose levothyroxine in the preconception period under specialist guidance. Crucially, TSH elevation from TSHR mutations is not autoimmune and does not reflect thyroid tissue destruction; thyroid antibodies (TPO-Ab, TG-Ab) will typically be negative, which is a useful diagnostic clue.

Homozygous carriers (extremely rare) require definitive endocrinology evaluation for congenital hypothyroidism management, which is beyond the scope of this entry.

Interactions

W546X in compound heterozygosity with other TSHR inactivating mutations (such as A553T, P162A, L467P, C600R, and others catalogued in OMIM 275200) produces phenotypes ranging from severe congenital hypothyroidism to compensated subclinical states depending on the combined residual receptor function. TSHR mutations do not interact with autoimmune thyroid genes such as CTLA4 (rs3087243) or PTPN22 (rs2476601) — the mechanism is structural, not immunological.